Work vehicle and control method for same

ABSTRACT

A control unit has a connection determining unit and a motor switch control unit. The connection determining unit determines whether assistance from a third motor is required or not. The third motor is set to a connected state when the connection determining unit determines that assistance from the third motor is required. The motor switch control unit controls the motor switching mechanism so that the third motor is set to a disconnected state when the connection determining unit determines that assistance from the third motor is not required.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National stage application of InternationalApplication No. PCT/JP2014/079963, filed on Nov. 12, 2014. This U.S.National stage application claims priority under 35 U.S.C. §119(a) toJapanese Patent Application No. 2013-237341, filed in Japan on Nov. 15,2013, the entire contents of which are hereby incorporated herein byreference.

BACKGROUND

1. Field of the Invention

The present invention relates to a work vehicle, in particular, ahybrid-type work vehicle, and to a method for controlling the same.

2. Background Information

Recently, a hybrid-type work vehicle has been proposed that travelsusing driving power from an engine and driving power from a motor. Ahydraulic-mechanical transmission (HMT) or an electric-mechanicaltransmission (EMT) is disclosed as a power transmission device forhybrid-type work vehicles as in, for example, Japanese Unexamined PatentApplication Publication No. 2006-329244.

The HMT has a planetary gear mechanism, and a first pump/motor and asecond pump/motor connected to rotating elements of the planetary gearmechanism. The first pump/motor and the second pump/motor function aseither hydraulic motors or hydraulic pumps in response to the travelstate of the work vehicle. The HMT is configured to enable steplesschanging of the rotation speed of the output shaft by changing therotation speed of the pump/motors. The HMT has a third pump/motor. Thethird pump/motor is provided to assist either the first pump/motor orthe second pump/motor. The pump/motors assisted by the third pump/motorare switched in response to the speed range of the vehicle speed.

An electric motor is used in the EMT in place of the hydraulic motor inthe HMT. That is, the EMT has a first generator/motor, a secondgenerator/motor, and a third generator/motor. The first and secondgenerator/motors function as either electric motors or electricgenerators in response to the travel state of the work vehicle. Thethird generator/motor is provided to assist either the firstgenerator/motor or the second generator/motor in the same way as theabove-mentioned third pump/motor. Similar to the HMT, the EMT isconfigured to enable stepless changing of the rotation speed of theoutput shaft by changing the rotation speed of the generator/motors.

SUMMARY

The maximum torque required for each motor may be reduced due to thethird motor assisting the first motor or the second motor, as in the HMTor the EMT above-mentioned. As a result, the size of the motors can bereduced.

The third motor in the HMT or the EMT is normally connected to eitherone of the first motor and the second motor. Therefore, the third motoris connected to the first motor or the second motor even when the torqueof the first motor or the second motor is sufficient. As a result, amechanical loss occurring in the third motor leads to a reduction in thepower transmission efficiency of the power transmission device.

An object of the present invention is to provide a hybrid-type workvehicle in which the power transmission efficiency of the powertransmission device is improved, and to provide a control method of thework vehicle.

A work vehicle according to a first embodiment of the present inventionis equipped with an engine, a hydraulic pump, a work implement, a traveldevice, a power transmission device, and a control unit. The hydraulicpump is driven by the engine. The work implement is driven by hydraulicfluid discharged from the hydraulic pump. The travel device is driven bythe engine. The power transmission device transmits driving power fromthe engine to the travel device. The control unit controls the powertransmission device. The power transmission device has an input shaft,an output shaft, a gear mechanism, a first motor, a second motor, athird motor, and a motor switching mechanism. The gear mechanism has aplanetary gear mechanism and transmits the rotation of the input shaftto the output shaft. The first motor is connected to first rotatingelements of the planetary gear mechanism. The second motor is connectedto second rotating elements of the planetary gear mechanism. The thirdmotor assists the first motor and the second motor. The motor switchingmechanism is able to selectively switch between a connected state inwhich the third motor is connected to the first motor or the secondmotor, and a disconnected state in which the third motor is disconnectedfrom both the first motor and the second motor. The power transmissiondevice is configured to change the rotation speed ratio of the outputshaft with respect to the input shaft by changing the rotation speeds ofthe first motor, the second motor, and the third motor. The control unithas a connection determining unit and a motor switch control unit. Theconnection determining unit determines whether assistance from the thirdmotor is required or not. The motor switch control unit sets the thirdmotor to the connected state when the connection determining unitdetermines that assistance from the third motor is required. The motorswitch control unit controls the motor switching mechanism so that thethird motor is set to the disconnected state when the connectiondetermining unit determines that assistance from the third motor is notrequired.

In this case, the disconnected state in which the third motor isdisconnected from the first motor and the second motor is brought aboutwhen assistance from the third motor is not required. As a result, theoccurrence of loss in the third motor can be suppressed and the powertransmission efficiency of the power transmission device can beimproved.

The control unit preferably is further provided with a motor commandvalue determining unit. The motor command value determining unitdetermines a command value for the torque or the rotation speed of thethird motor. When the third motor is in the disconnected state, themotor command value determining unit sets the command value for thetorque or the rotation speed of the third motor to a predetermined firststandby command value.

In this case, loss in the third motor can be further suppressed bysetting the predetermined first standby command value to zero or a smallvalue.

When the third motor is switched from the disconnected state to theconnected state, the motor command value determining unit preferablydetermines the command value for the third motor so that the rotationspeed of the third motor is synchronized with the rotation speed of themotor to be connected with the third motor among the first motor and thesecond motor.

In this case, the connection between the third motor and the first motoror the second motor can be carried out smoothly. As a result, theoccurrence of a shock during connection can be suppressed.

The motor switching mechanism preferably has a clutch. The clutchswitches between connection and disconnection of the third motor withthe first motor or the second motor. The third motor is switched fromthe disconnected state to the connected state due to the clutchswitching from the disconnected state to the connected state. Thecontrol unit further has a predicted speed computing unit. The predictedspeed computing unit computes a predicted rotation speed. The predictedrotation speed is a predicted value of the rotation speed of the firstmotor or the second motor after a predetermined first predicted timeperiod has elapsed from the current point in time. The motor switchcontrol unit starts the connection of the clutch when the differencebetween the rotation speed of the predicted rotation speed equivalent tothe rotating shaft of the third motor and the rotation speed of thethird motor is equal to or less than a predetermined switchingthreshold.

In this case, the third motor can be switched from the disconnectedstate to the connected state at a timing when the rotation speed of thefirst motor or the second motor equivalent to the rotating shaft of thethird motor approaches the rotation speed of the third motor inconsideration of the time required for switching the clutch. As aresult, the occurrence of a shock during connection can be suppressed.

The motor command determining unit preferably sets a command torque forthe third motor to a predetermined standby command value until apredetermined second predicted time period from a connection startingtime point of the clutch has elapsed. In this case, the command torquefor the third motor becomes the predetermined standby command valueuntil the connection of the clutch is complete. In this case, loss inthe third motor can be reduced by setting the predetermined standbycommand value to zero or a small value.

The motor command determining unit preferably determines a requiredcommand torque for the motor connected with the third motor. When thesecond predicted time period from the connection starting time point ofthe clutch has elapsed, the motor command determining unit determinesthe command torques for the motor connected to the third motor and forthe third motor on the basis of the required command torque. In thiscase, the assistance from the third motor can be started at a timingapproaching the completion time point of the clutch connection.

The work vehicle preferably is further equipped with a clutchreplenishment detecting unit. The clutch replenishment detecting unitdetects when the replenishment of hydraulic fluid to the clutch iscompleted. The motor command determining unit sets the command torquefor the third motor to the predetermined standby command value from theconnection starting time point of the clutch until the clutchreplenishment detecting unit detects that the replenishment iscompleted. In this case, the command torque for the third motor becomesthe predetermined standby command value until the connection of theclutch is completed. In this case, loss in the third motor can bereduced by setting the predetermined standby command value to zero or asmall value.

The motor command determining unit preferably determines a requiredcommand torque for the motor connected with the third motor. When theclutch replenishment detecting unit detects that the replenishment iscompleted after the start of the clutch connection, the motor commanddetermining unit determines the command torques for the motor connectedto the third motor and for the third motor on the basis of the requiredcommand torque. In this case, the assistance from the third motor can bestarted at a timing approaching the time point that the clutchconnection is completed.

The work vehicle is preferably further provided with a speed changeoperating member. The speed change operating member is a member forselecting a speed range that defines an upper limit of the vehiclespeed. The connection determining unit determines that the assistancefrom the third motor is required when a predetermined connectiondetermination condition is met. The predetermined connectiondetermination condition includes the selection of the lowest speed rangeby the speed change operating member.

In this case, the third motor is set to the connected state when thelowest speed range is selected when the operator requires a largetorque. As a result, a large torque can be obtained due to the thirdmotor assisting the first motor or the second motor.

The work vehicle preferably further has a vehicle speed detecting unitfor detecting the vehicle speed and an accelerator operating member. Thecontrol unit further has a transmission requirement determination unit.The transmission requirement determination unit determines the requiredtractive force of the power transmission device on the basis of thevehicle speed and the operating amount of the accelerator operatingmember. The connection determining unit determines that the assistancefrom the third motor is required when the predetermined connectiondetermination condition is met. The predetermined connectiondetermination condition includes the tractive force of the powertransmission device obtained from the output torque of the first motorand the output torque of the second motor being less than the requiredtractive force.

In this case, the third motor is set to the connected state when theassistance from the third motor is required for obtaining the requiredtractive force. As a result, the required tractive force can be obtaineddue to the third motor assisting the first motor or the second motor.

The work vehicle preferably further has a speed change operating member,a vehicle speed detecting unit, and an accelerator operating member. Thespeed change operating member is a member for selecting a speed rangethat defines an upper limit of the vehicle speed. The vehicle speeddetecting unit detects the vehicle speed. The control unit further has atransmission requirement determination unit. The transmissionrequirement determination unit determines the required tractive force ofthe power transmission device on the basis of the vehicle speed and theoperating amount of the accelerator operating member. The connectiondetermining unit determines that the assistance from the third motor isnot required when a predetermined disconnection determination conditionis met. The predetermined disconnection determination condition includesthe selection of a speed range other than the lowest speed range by thespeed change operating member and the tractive force obtained from theoutput torque of the first motor and the output torque of the secondmotor not being less than the required tractive force.

In this case, the third motor is set to the disconnected state when aspeed range other than the lowest speed range is selected and thetractive force obtained from the output torque of the first motor andthe output torque of the second motor is not less than the requiredtractive force when the operator does not require a large torque. As aresult, the occurrence of loss with the third motor can be suppressedand fuel consumption can be improved.

The motor switch control unit preferably controls the motor switchingmechanism so that the third motor is set to the disconnected state whenthe rotation speed of the third motor is greater than a predeterminedupper limit value. In this case, the third motor can be protected bypreventing over-rotating of the third motor.

When the rotation speed of the third motor after the third motor is setto the connected state exceeds a predetermined upper limit rotationspeed, the motor switch control unit preferably switches the third motorto the disconnected state and a motor command value determining unitsets the command value of the rotation speed of the third motor to apredetermined second standby command value that is greater than thefirst standby command value. In this case, the third motor can beconnected more quickly by setting the third motor to be on standby withthe second standby command value.

The connection determining unit preferably determines that the tractiveforce obtained from the output torque of the first motor and the outputtorque of the second motor is less than the required tractive force whenthe output torque of the first motor or the output torque of the secondmotor required for generating the required tractive force are equal toor greater than a predetermined upper limit threshold. As a result, thefact that the tractive force is less than the required tractive forcecan be determined more accurately.

The work vehicle is preferably further provided with a mode switchingmechanism and a speed change operating member. The mode switchingmechanism is a mechanism for selectively switching the driving powerdrivetrain of the power transmission device among a plurality of modes.The speed change operating member is a member for selecting a speedrange that defines an upper limit of the vehicle speed. When the thirdmotor is in the disconnected state, the motor command value determiningunit determines the command values for the torque or the rotation speedof the third motor so that the third motor is set to a standby state ata predetermined rotation speed according to the selected mode and thespeed range. In this case, the third motor is put on standby at arotation speed conforming to the selected mode and the speed range. Forexample, when the rotation speed of the motor connected with the thirdmotor is predicted to be high based on the selected mode and the speedrange, the third motor is put on standby at a high rotation speedwhereby the third motor can be connected quickly.

A control method for a work vehicle according to another exemplaryembodiment of the present invention is a control method for a workvehicle equipped with an engine, a hydraulic pump, a work implement, atravel device, and a power transmission device. The hydraulic pump isdriven by the engine. The work implement is driven by hydraulic fluiddischarged from the hydraulic pump. The travel device is driven by theengine. The power transmission device transmits driving power from theengine to the travel device. The power-transmission device has an inputshaft, an output shaft, a gear mechanism, a first motor, a second motor,a third motor, and a motor switching mechanism. The gear mechanism has aplanetary gear mechanism and transmits the rotation of the input shaftto the output shaft. The first motor is connected to the first rotatingelements of the planetary gear mechanism. The second motor is connectedto the second rotating elements of the planetary gear mechanism. Thethird motor assists the first motor and the second motor. The motorswitching mechanism is able to selectively switch between a connectedstate in which the third motor is connected to the first motor or thesecond motor, and a disconnected state in which the third motor isdisconnected from the first motor and the second motor. The powertransmission device is configured to change the rotation speed ratio ofthe output shaft with respect to the input shaft by changing therotation speeds of the first motor, the second motor, and the thirdmotor. The control method according to the present exemplary embodimentincludes a first step, a second step, and a third step. The need forassistance from the third motor is determined in the first step. Whenassistance from the third motor is determined to be required, the motorswitching mechanism controls the third motor so that the third motor isset to the connected state in the second step. The motor switchingmechanism controls the third motor so that the third motor is set to thedisconnected state when assistance from the third motor is determined asnot being required in the third step.

According to the present invention, a hybrid-type work vehicle thatallows the power transmission efficiency of the power transmissiondevice to be improved can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side view of a work vehicle according to an exemplaryembodiment of the present invention.

FIG. 2 is a schematic view of a configuration of the work vehicle.

FIG. 3 is a schematic view of a configuration of a power transmissiondevice.

FIG. 4 illustrates changes in rotation speeds of a first motor, a secondmotor, and a third motor with respect to the rotation speed ratio.

FIG. 5 is a control block diagram illustrating processing executed bythe control unit.

FIG. 6 is a control block diagram illustrating processing executed bythe control unit.

FIG. 7 is a flow chart illustrating processing when the third motor isswitched between the disconnected state and the connected state.

FIG. 8 is a flow chart illustrating a method for determining a secondconnection determination condition.

FIG. 9 is a flow chart illustrating processing for controllingsynchronization with the third motor.

FIG. 10 illustrates changes between a first rotation speed, a firstpredicted rotation speed, and the rotation speed of the third motorduring synchronization control.

FIG. 11 is a flow chart illustrating processing when the third motor isswitched from the connected state to the disconnected state.

FIG. 12 illustrates changes in the rotation speeds of the first motor,the second motor, and the third motor when the third motor is put onstandby in a first standby mode and a second standby mode.

FIG. 13 is a flow chart illustrating processing when the connectiontarget of the third motor is switched from the second motor to the firstmotor.

FIG. 14 illustrates changes between the first rotation speed and thesecond rotation speed during switching control.

FIG. 15 illustrates distribution of the command torques to the firstmotor and the third motor with respect to the required torque.

FIG. 16 illustrates changes in rotation speeds and command torques forthe first motor, the second motor, and the third motor in the presentembodiment.

FIG. 17 illustrates changes in rotation speeds and command torques ofthe first motor and the second motor with respect to the vehicle speedratio in a comparative example.

FIG. 18 is a control block diagram illustrating processing executed bythe control unit according to another exemplary embodiment.

FIG. 19 illustrates distribution of the command torques for the firstmotor and the third motor according to a first modified example.

FIG. 20 illustrates distribution of the command torques for the firstmotor and the third motor according to a second modified example.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Exemplary embodiments of the present invention will be explained indetail with reference to the figures. FIG. 1 is a side view of a workvehicle 1 according to an exemplary embodiment of the present invention.As illustrated in FIG. 1, the work vehicle 1 is equipped with a vehiclebody frame 2, a work implement 3, traveling wheels 4 and 5, and anoperating cabin 6. The work vehicle 1 is a wheel loader and travels dueto the traveling wheels 4 and 5 being rotated and driven. The workvehicle 1 is able to carry out work, such as excavation, by using thework implement 3.

The work implement 3 and the traveling wheels 4 and 5 are attached tothe vehicle body frame 2. The work implement 3 is driven by hydraulicfluid from a below-mentioned work implement pump 23 (see FIG. 2). Thework implement 3 has a boom 11 and a bucket 12. The boom 11 is mountedon the vehicle body frame 2. The work implement 3 includes a liftcylinder 13 and a bucket cylinder 14. The lift cylinder 13 and thebucket cylinder 14 are hydraulic cylinders. One end of the lift cylinder13 is attached to the vehicle body frame 2. The other end of the liftcylinder 13 is attached to the boom 11. The boom 11 swings up and downdue to the extension and contraction of the lift cylinder 13 due tohydraulic fluid from the work implement pump 23. The bucket 12 isattached to the tip of the boom 11. One end of the bucket cylinder 14 isattached to the vehicle body frame 2. The other end of the bucketcylinder 14 is attached to the bucket 12 via a bell crank 15. The bucket12 swings up and down due to the extension and contraction of the bucketcylinder 14 due to hydraulic fluid from the work implement pump 23.

The operating cabin 6 is attached to the vehicle body frame 2. Theoperating cabin 6 is mounted on the vehicle body frame 2. A seat for theoperator and a below-mentioned operating device are disposed in theoperating cabin 6. The vehicle body frame 2 has a front frame 16 and arear frame 17. The front frame 16 and the rear frame 17 are attached toeach other in a manner that allows swinging in the left-right direction.

The work vehicle 1 has a steering cylinder 18. The steering cylinder 18is attached to the front frame 16 and the rear frame 17. The steeringcylinder 18 is a hydraulic cylinder. The work vehicle 1 is able tochange the advancing direction to the right and left with the extensionand contraction of the steering cylinder 18 due to hydraulic fluid froma below-mentioned steering pump 30.

FIG. 2 is a schematic view of a configuration of the work vehicle 1. Asillustrated in FIG. 2, the work vehicle 1 is equipped with an engine 21,a power take-off (PTO) 22, a power transmission device 24, a traveldevice 25, an operating device 26, and a control unit 27.

The engine 21 is, for example, a diesel engine. The output of the engine21 is controlled by adjusting the amount of fuel injected into thecylinders of the engine 21. The adjustment of the amount of fuel isconducted by the control unit 27 controlling a fuel injection device 28attached to the engine 21. The work vehicle 1 is equipped with an enginerotation speed detecting unit 31. The engine rotation speed detectingunit 31 detects the engine rotation speed and transmits a detectionsignal indicating the engine rotation speed to the control unit 27.

The work vehicle 1 has the work implement pump 23, the steering pump 30,and a transmission pump 29. The work implement pump 23, the steeringpump 30, and the transmission pump 29 are hydraulic pumps. The PTO 22(power take-off) transmits a portion of the driving power from theengine 21 to the hydraulic pumps 23, 30, and 29. That is, the PTO 22distributes the driving power from the engine 21 to the powertransmission device 24 and the hydraulic pumps 23, 30, and 29.

The work implement pump 23 is driven by driving power from the engine21. The hydraulic fluid discharged from the work implement pump 23 issupplied to the lift cylinder 13 and the bucket cylinder 14 through awork implement control valve 41. The work vehicle 1 is equipped with awork implement pump pressure detecting unit 32. The work implement pumppressure detecting unit 32 detects a discharge pressure (referred tobelow as “work implement pump pressure”) of hydraulic fluid from thework implement pump 23 and transmits a detection signal indicating thework implement pump pressure to the control unit 27.

The work implement pump 23 is a variable displacement hydraulic pump.The discharge capacity of the work implement pump 23 is changed bychanging the tilt angle of a skew plate or an inclined shaft of the workimplement pump 23. A first capacity control device 42 is connected tothe work implement pump 23. The first capacity control device 42 iscontrolled by the control unit 27 and changes the tilt angle of the workimplement pump 23. As a result, the discharge capacity of the workimplement pump 23 is controlled by the control unit 27. The work vehicle1 is equipped with a first tilt angle detecting part 33. The first tiltangle detecting part 33 detects the tilt angle of the work implementpump 23 and transmits a detection signal indicating the tilt angle tothe control unit 27.

The steering pump 30 is driven by driving power from the engine 21. Thehydraulic fluid discharged from the steering pump 30 is supplied to theabove-mentioned steering cylinder 18 through a steering control valve43. The work vehicle 1 is equipped with a steering pump pressuredetecting unit 34. The steering pump pressure detecting unit 34 detectsthe discharge pressure (referred to below as “steering pump pressure”)of hydraulic fluid from the steering pump 30 and transmits a detectionsignal indicating the steering pump pressure to the control unit 27.

The steering pump 30 is a variable displacement hydraulic pump. Thedischarge capacity of the steering pump 30 is changed by changing thetilt angle of a skew plate or an inclined shaft of the steering pump 30.A second capacity control device 44 is connected to the steering pump30. The second capacity control device 44 is controlled by the controlunit 27 and changes the tilt angle of the steering pump 30. As a result,the discharge capacity of the steering pump 30 is controlled by thecontrol unit 27. The work vehicle 1 is equipped with a second tilt angledetecting part 35. The second tilt angle detecting part 35 detects thetilt angle of the steering pump 30 and transmits a detection signalindicating the tilt angle to the control unit 27.

The transmission pump 29 is driven by driving power from the engine 21.The transmission pump 29 is a fixed displacement hydraulic pump. Thehydraulic fluid discharged from the transmission pump 29 is supplied tobelowmentioned clutches CF, CR, CL, CH, Cm1, and Cm2 of the powertransmission device 24 via a clutch control valve 38.

The PTO 22 transmits a portion of the driving power from the engine 21to the power transmission device 24. The power transmission device 24transmits the driving power from the engine 21 to the travel device 25.The power transmission device 24 changes the speed in the driving powerfrom the engine 21 and outputs it. An explanation of the configurationof the power transmission device 24 is provided in detail below.

The travel device 25 has an axle 45 and the traveling wheels 4 and 5.The axle 45 transmits driving power from the power transmission device24 to the traveling wheels 4 and 5. As a result, the traveling wheels 4and 5 rotate. The work vehicle 1 is equipped with a vehicle speeddetecting unit 37. The vehicle speed detecting unit 37 detects therotation speed (referred to below as “output rotation speed”) of anoutput shaft 63 of the power transmission device 24. The output rotationspeed corresponds to the vehicle speed and consequently the vehiclespeed detecting unit 37 detects the vehicle speed by detecting theoutput rotation speed. The vehicle speed detecting unit 37 detects therotating direction of the output shaft 63. The rotating direction of theoutput shaft 63 corresponds to the traveling direction of the workvehicle 1 and consequently the vehicle speed detecting unit 37 functionsas a traveling direction detecting unit that detects the travelingdirection of the work vehicle 1 by detecting the rotating direction ofthe output shaft 63. The vehicle speed detecting unit 37 transmitsdetection signals indicating the output rotation speed and the rotatingdirection to the control unit 27.

The operating device 26 is operated by an operator. The operating device26 has an accelerator operating device 51, a work implement operatingdevice 52, a speed change operating device 53, a forward/reverse traveloperating device 54 (referred to below as “FR operating device 54”), asteering operating device 57, and a brake operating device 58.

The accelerator operating device 51 has an accelerator operating member51 a and an accelerator operation detecting unit 51 b. The acceleratoroperating member 51 a is operated in order to set a target rotationspeed of the engine 21. The accelerator operation detecting unit 51 bdetects an operating amount (referred to below as “accelerator operatingamount”) of the accelerator operating member 51 a. The acceleratoroperation detecting unit 51 b transmits a detection signal indicatingthe accelerator operating amount to the control unit 27.

The work implement operating device 52 has a work implement operatingmember 52 a and a work implement operation detecting unit 52 b. The workimplement operating member 52 a is operated in order to actuate the workimplement 3. The work implement operation detecting unit 52 b detects aposition of the work implement operating member 52 a. The work implementoperation detecting unit 52 b outputs a detection signal indicating theposition of the work implement operating member 52 a to the control unit27. The work implement operation detecting unit 52 b detects anoperating amount of the work implement operating member 52 a bydetecting a position of the work implement operating member 52 a.

The speed change operating device 53 has a speed change operating member53 a and a speed change operation detecting unit 53 b. The speed changeoperating member 53 a is a member for selecting a speed range thatdefines an upper limit of the vehicle speed. The operator is able toselect a speed range of the power transmission device 24 by operatingthe speed change operating member 53 a. The speed change operationdetecting unit 53 b detects a position of the speed change operatingmember 53 a. The position of the speed change operating member 53 acorresponds to a plurality of speed ranges such as a first speed and asecond speed and the like. The speed change operation detecting unit 53b outputs a detection signal indicating the position of the speed changeoperating member 53 a to the control unit 27.

The FR operating device 54 has a forward/reverse travel operating member54 a (referred to below as “FR operating member 54 a”) and aforward/reverse travel position detecting unit 54 b (referred to belowas a “FR position detecting unit 54 b”). The operator can switch betweenforward and reverse travel of the work vehicle 1 by operating the FRoperating member 54 a. The FR operating member 54 a is selectivelyswitched between a forward travel position (F), a neutral position (N),and a reverse travel position (R). The FR position detecting unit 54 bdetects a position of the FR operating member 54 a. The FR positiondetecting unit 54 b outputs a detection signal indicating the positionof the FR operating member 54 a to the control unit 27.

The steering operating device 57 has a steering operating member 57 a.The steering operating device 57 drives the steering control valve 43 bysupplying pilot hydraulic pressure based on an operation of the steeringoperating member 57 a to the steering control valve 43. The steeringoperating device 57 may drive the steering control valve 43 byconverting an operation of the steering operating member 57 a to anelectrical signal. The operator is able to change the travel directionof the work vehicle 1 to the right or left by operating the steeringoperating member 57 a.

The brake operating device 58 has a brake operating member 58 a and abrake operation detecting unit 58 b. The operator is able to operate adeceleration force of the work vehicle 1 by operating the brakeoperating member 58 a. The brake operation detecting unit 58 b detectsan operating amount of the brake operating member 58 a (referred tobelow as “brake operating amount”). The brake operation detecting unit58 b outputs a detection signal indicating the brake operating amount tothe control unit 27. The pressure of the brake oil may be used as thebrake operating amount.

The control unit 27 has a calculation device, such as a CPU, and amemory, such as a RAM or a ROM, and conducts processing for controllingthe work vehicle 1. The control unit 27 has a storage unit 56. Thestorage unit 56 stores programs and data for controlling the workvehicle 1.

The control unit 27 transmits a command signal indicating a commandthrottle value to the fuel injection device 28 so that a target rotationspeed of the engine 21 is obtained in accordance with the acceleratoroperating amount. The control of the engine 21 by the control unit 27 isdescribed in detail below.

The control unit 27 controls hydraulic pressure supplied to thehydraulic cylinders 13 and 14 by controlling the work implement controlvalve 41 on the basis of the detection signals from the work implementoperation detecting unit 52 b. As a result, the hydraulic cylinders 13and 14 expand or contract to operate the work implement 3.

The control unit 27 controls the power transmission device 24 on thebasis of the detection signals from each of the detecting units. Thecontrol of the power transmission device 24 by the control unit 27 isdescribed in detail below.

An explanation of the configuration of the power transmission device 24is provided in detail below. FIG. 3 is a schematic view of aconfiguration of the power transmission device 24. As illustrated inFIG. 3, the power transmission device 24 is provided with an input shaft61, a gear mechanism 62, the output shaft 63, a first motor MG1, asecond motor MG2, a third motor MG3, and a capacitor 64. The input shaft61 is connected to the abovementioned PTO 22. The rotation from theengine 21 is inputted to the input shaft 61 via the PTO 22. The gearmechanism 62 transmits the rotation of the input shaft 61 to the outputshaft 63. The output shaft 63 is connected to the abovementioned traveldevice 25, and transmits the rotation from the gear mechanism 62 to theabove-mentioned travel device 25.

The gear mechanism 62 is a mechanism for transmitting driving power fromthe engine 21. The gear mechanism 62 is configured so that the rotationspeed ratio of the output shaft 63 with respect to the input shaft 61 ischanged in response to changes in the rotation speeds of the motors MG1,MG2, and MG3. The gear mechanism 62 has a FR switch mechanism 65, and aspeed change mechanism 66.

The FR switch mechanism 65 has the forward travel clutch CF (referred tobelow as “F-clutch CF”), the reverse travel clutch CR (referred to belowas “R-clutch CR”), and various other gears not illustrated. The F-clutchCF and the R-clutch CR are hydraulic clutches and hydraulic fluid issupplied from the transmission pump 29 to the clutches CF and CR. Thehydraulic fluid for the F-clutch CF is controlled by the clutch controlvalve 38 illustrated in FIG. 2. The hydraulic fluid for the R-clutch CRis controlled by the clutch control valve 38. The clutch control valve38 is controlled by command signals from the control unit 27.

The direction of the rotation outputted from the FR switch mechanism 65is switched due to the switching between connected/disconnected statesof the F-clutch CF and disconnected/connected states of the R-clutch CR.Specifically, the F-clutch CF is connected and the R-clutch CR isdisconnected when the vehicle is traveling forward. The F-clutch CF isdisconnected and the R-clutch CR is connected when the vehicle istraveling in reverse.

The speed change mechanism 66 has a transmission shaft 67, a firstplanetary gear mechanism 68, a second planetary gear mechanism 69, aHi/Lo switch mechanism 70, and an output gear 71. The transmission shaft67 is coupled to the FR switch mechanism 65. The first planetary gearmechanism 68 and the second planetary gear mechanism 69 are disposed onthe same axis as the transmission shaft 67.

The first planetary gear mechanism 68 has a first sun gear S1, aplurality of first planet gears P1, a first carrier C1 that supports theplurality of first planet gears P1, and a first ring gear R1. The firstsun gear S1 is coupled to the transmission shaft 67. The plurality offirst planet gears P1 mesh with the first sun gear S1 and are supportedin a rotatable manner by the first carrier C1. A first carrier gear Gc1is provided on an outer peripheral part of the first carrier C1. Thefirst ring gear R1 meshes with the plurality of first planet gears P1and is able to rotate. A first ring outer periphery gear Gr1 is providedon the outer periphery of the first ring gear R1.

The second planetary gear mechanism 69 has a second sun gear S2, aplurality of second planet gears P2, a second carrier C2 that supportsthe plurality of second planet gears P2, and a second ring gear R2. Thesecond sun gear S2 is coupled to the first carrier C1. The plurality ofsecond planet gears P2 mesh with the second sun gear S2 and aresupported in a rotatable manner by the second carrier C2. The secondring gear R2 meshes with the plurality of second planet gears P2 and isable to rotate. A second ring outer periphery gear Gr2 is provided onthe outer periphery of the second ring gear R2. The second ring outerperiphery gear Gr2 meshes with the output gear 71, and the rotation ofthe second ring gear R2 is outputted to the output shaft 63 via theoutput gear 71.

The Hi/Lo switch mechanism 70 is a mechanism for switching the drivingpower drivetrain of the power transmission device 24 between ahigh-speed mode (Hi-mode) in which the vehicle speed is high and alow-speed mode (Lo mode) in which the vehicle speed is low. The Hi/Loswitch mechanism 70 has the H-clutch CH that is connected during theHi-mode and the L-clutch CL that is connected during the Lo mode. TheH-clutch CH connects or disconnects the first ring gear R1 and thesecond carrier C2. The L-clutch CL connects or disconnects the secondcarrier C2 and a fixed end 72 to prohibit or allow the rotation of thesecond carrier C2.

The clutches CH and CL are hydraulic clutches, and hydraulic fluid fromthe transmission pump 29 is supplied to each of the clutches CH and CL.The hydraulic fluid for the H-clutch CH is controlled by the clutchcontrol valve 38. The hydraulic fluid for the L-clutch CL is controlledby the clutch control valve 38.

The first motor MG1 and the second motor MG2 function as drive motorsthat generate driving power using electrical energy. The first motor MG1and the second motor MG2 also function as generators that use inputteddriving power to generate electrical energy. The first motor MG1functions as a generator when a command signal from the control unit 27is applied so as to activate a torque in the reverse direction of therotating direction of the first motor MG1. A first motor gear Gm1 isfixed to a rotating shaft Sm1 of the first motor MG1 and the first motorgear Gm1 meshes with the first carrier gear Gc1.

The second motor MG2 is configured in the same way as the first motorMG1. A second motor gear Gm2 is fixed to a rotating shaft Sm2 of thesecond motor MG2 and the second motor gear Gm2 meshes with the firstring outer periphery gear Gr1.

The third motor MG3 assists the first motor MG1 and the second motorMG2. The third motor MG3 is configured in the same way as the firstmotor MG1 and the second motor MG2. The speed change mechanism 66 has amotor switching mechanism 73, and the motor switching mechanism 73selectively switches the target of the assistance from the third motorMG3 to the first motor MG1 or the second motor MG2.

Specifically, the motor switching mechanism 73 has the first motorclutch Cm1, the second motor clutch Cm2, a first connecting gear Ga1,and a second connecting gear Ga2. A third motor gear Gm3 is connected toa rotating shaft Sm3 of the third motor MG3, and the third motor gearGm3 meshes with the first connecting gear Ga1. The first motor clutchCm1 switches between connecting and disconnecting the first connectinggear Ga1 and the rotating shaft Sm1 of the first motor MG1. The firstconnecting gear Ga1 meshes with the second connecting gear Ga2. Thesecond motor clutch Cm2 switches between connecting and disconnectingthe second connecting gear Ga2 and the rotating shaft Sm2 of the secondmotor MG2.

The first motor clutch Cm1 and the second motor clutch Cm2 are hydraulicclutches. Hydraulic oil from the transmission pump 29 is supplied toeach of the motor clutches Cm1 and Cm2. The hydraulic fluid for themotor clutches Cm1 and Cm2 is controlled by the clutch control valve 38.

The motor switching mechanism 73 is able to switch the third motor MG3between a first connected state, a second connected state, and adisconnected state. The first motor clutch Cm1 is connected and thesecond motor clutch Cm2 is disconnected in the first connected state.That is, the third motor MG3 is connected to the first motor MG1 andassists the first motor MG1 in the first connected state. The secondmotor clutch Cm2 is connected and the first motor clutch Cm1 isdisconnected in the second connected state. That is, the third motor MG3is connected to the second motor MG2 and assists the second motor MG2 inthe second connected state. The first motor clutch Cm1 and the secondmotor clutch Cm2 are disconnected in the disconnected state. That is,the third motor MG3 is disconnected from the first motor MG1 and thesecond motor MG2 and does not assist the first motor MG1 and the secondmotor MG2 in the disconnected state.

The first motor MG1 is connected to the capacitor 64 via a firstinverter I1. The second motor MG2 is connected to the capacitor 64 via asecond inverter I2. The third motor MG3 is connected to the capacitor 64via a third inverter I3.

The capacitor 64 functions as an energy reservoir unit for storingenergy generated by the motors MG1, MG2, and MG3. That is, the capacitor64 stores electrical power generated in the motors MG1, MG2, and MG3when the total electrical power generation amount of the motors MG1 andMG2 is high. The capacitor 64 releases electrical power when the totalelectrical power consumption amount of the motors MG1, MG2, and MG3 ishigh. That is, the motors MG1, MG2, and MG3 are driven by electricalpower stored in the capacitor 64. Alternatively, the motors MG1, MG2,and MG3 can drive using the electrical power stored in the capacitor 64.A battery may be used as another electrical power storage means in placeof the capacitor. Further, a boosting transformer may be provided foradjusting the voltage between the electrical power storage means and theinverters.

The control unit 27 receives detection signals from the variousdetecting units and applies command signals for indicating the commandtorques for the motors MG1, MG2, and MG3 to inverters I1, I2, and I3.The control unit 27 may output rotation speed commands to the motorsMG1, MG2, and MG3. In this case, the inverters I1, I2, and I3 controlthe motors MG1, MG2, and MG3 by calculating command torquescorresponding to the rotation speed commands. The control unit 27applies the command signals for controlling the clutch hydraulicpressure of the clutches CF, CR, CH, CL, Cm1, and Cm2 to the clutchcontrol valve 38. The clutch control valve 38 includes a plurality ofvalves for controlling the clutches CF, CR, CH, CL, Cm1, and Cm2.

The speed change ratio and the output torque of the power transmissiondevice 24 are controlled by controlling the motors MG1, MG2, and MG3 andthe clutches CF, CR, CH, CL, Cm1, and Cm2 with command signals from thecontrol unit 27. The following is an explanation of the operations ofthe power transmission device 24.

An outline of operations of the power transmission device 24 when thevehicle speed increases from zero in the forward travel side while therotation speed of the engine 21 remains fixed, will be explained withreference to FIG. 4. That is, FIG. 4 illustrates the relationshipbetween the rotation speeds of the motors MG1, MG2, and MG3 and therotation speed ratio of the power transmission device 24. When therotation speed of the engine 21 is fixed, the vehicle speed changes inresponse to the rotation speed ratio of the power transmission device24. The rotation speed ratio is the ratio of the rotation speed of theoutput shaft 63 with respect to the rotation speed of the input shaft61. Therefore, the changes in the vehicle speed in FIG. 4 match thechanges in the rotation speed ratio of the power transmission device 24.That is, FIG. 4 illustrates the rotation speeds of the motors MG1, MG2,and MG3 with respect to the vehicle speed. A long dashed short dashedline La_m1 in FIG. 4 depicts the rotation speed of the first motor MG1,and a long dashed short dashed line La_m2 depicts the rotation speed ofthe second motor MG2. A solid line La_m3 and a dashed line La_m3′ depictthe rotation speed of the third motor MG3. The rotation speeds of themotors MG1, MG2, and MG3 on the vertical axis in FIG. 4 may be a ratioof the rotation speeds of the motors MG1, MG2, and MG3 with respect tothe rotation speed of the engine 21.

FIG. 4 further illustrates the connected (ON) states and thedisconnected (OFF) states of the first motor clutch Cm1 and the secondmotor clutch Cm2. A solid line La_cm1 and a dashed line La_cm1′ depictchanges in the command signals to the first motor clutch Cm1 in FIG. 4.A solid line La_cm2 depicts changes in the command signals to the secondmotor clutch Cm2.

The driving power drivetrain of the power transmission device 24 is inthe Lo-mode when the rotation speed ratio is a value between zero and apredetermined threshold RSTh1. The L-clutch CL is connected and theH-clutch CH is disconnected in the Lo-mode. Because the H-clutch CH isdisconnected in the Lo-mode, the second carrier C2 and the first ringgear R1 are disconnected. Because the L-clutch CL is connected, thesecond carrier C2 is fixed.

The driving power from the engine 21 in the Lo-mode is inputted to thefirst sun gear S1 via the transmission shaft 67, and the driving poweris outputted from the first carrier C1 to the second sun gear S2.Conversely, the driving power inputted to the first sun gear S1 istransmitted from the first planet gears P1 to the first ring gear R1 andoutputted through the first ring outer periphery gear Gr1 and the secondmotor gear Gm2 to the second motor MG2. The second motor MG2 functionsmainly as a generator in the Lo-mode, and a portion of the electricalpower generated by the second motor MG2 is stored in the capacitor 64. Aportion of the electrical power generated by the second motor MG2 isconsumed in the driving of the first motor MG1.

The first motor MG1 functions mainly as an electric motor in theLo-mode. The driving power of the first motor MG1 is outputted to thesecond sun gear S2 along a path from the first motor gear Gm1 to thefirst carrier gear Gc1 to the first carrier C1. At this time, theelectrical power for driving the first motor MG1 is supplied from thesecond motor MG2 or from the capacitor 64 as needed. The driving poweroutputted to the second sun gear S2 as described above is transmitted tothe output shaft 63 along a path from the second planet gear P2 to thesecond ring gear R2 to the second ring outer periphery gear Gr2 to theoutput gear 71.

When assistance from the third motor MG3 is determined to be required inthe Lo-mode, the third motor MG3 is connected to the first motor MG1 orto the second motor MG2. When assistance from the third motor MG3 isdetermined as not being required, the third motor MG3 is disconnectedfrom the first motor MG1 and the second motor MG2 and is set to thedisconnected state. The first motor clutch Cm1 and the second motorclutch Cm2 are disconnected when the assistance from the third motor MG3is determined as not required. A solid line La_m3 in the Lo-mode in FIG.4 depicts the rotation speed of the third motor MG3 when assistance fromthe third motor MG3 is not required.

When the third motor MG3 assists the first motor MG1 in the Lo-mode, thefirst motor clutch Cm1 is connected and the second motor clutch Cm2 isdisconnected. Therefore, the first connecting gear Ga1 is connected tothe rotating shaft Sm3 of the first motor MG1 and the second connectinggear Ga2 is disconnected from the rotating shaft Sm2 of the second motorMG2. As a result, the third motor MG3 is connected to the first motorMG1 via the third motor gear Gm3, the first connecting gear Ga1, and thefirst motor clutch Cm1. The third motor MG3 is disconnected from thesecond motor MG2 because the second motor clutch Cm2 is disconnected. Adashed line La_m3′ in the Lo-mode in FIG. 4 depicts the rotation speedof the third motor MG3 when the first motor MG1 is assisted by the thirdmotor MG3.

When the third motor MG3 assists the second motor MG2 in the Lo-mode,the first motor clutch Cm1 is disconnected and the second motor clutchCm2 is connected. Therefore, the second connecting gear Ga2 is connectedto the rotating shaft Sm2 of the second motor MG2 and the firstconnecting gear Ga1 is disconnected from the rotating shaft Sm1 of thefirst motor MG1. As a result, the third motor MG3 is connected to thesecond motor MG2 via the third motor gear Gm3, the first connecting gearGa1, the second connecting gear Ga2, and the second motor clutch Cm2.The third motor MG3 is disconnected from the first motor MG1 because thefirst motor clutch Cm1 is disconnected.

The driving power drivetrain of the power transmission device 24 is inthe Hi-mode when the rotation speed ratio is a value that exceeds RSth1.The H-clutch CH is connected and the L-clutch CL is disconnected in theHi-mode. Because the H-clutch CH is connected in the Hi-mode, the secondcarrier C2 and the first ring gear R1 are connected. Because theL-clutch CL is disconnected, the second carrier C2 is disconnected.Therefore, the rotation speed of the first ring gear R1 and the secondcarrier C2 match.

The driving power from the engine 21 in the Hi-mode is inputted to thefirst sun gear S1 and the driving power is outputted from the firstcarrier C1 to the second sun gear S2. The driving power inputted to thefirst sun gear S1 is outputted from the first carrier C1 through thefirst carrier gear Gc1 and the first motor gear Gm1 to the first motorMG1. The first motor MG1 functions mainly as a generator in the Hi-mode,and thus a portion of the electrical power generated by the first motorMG1 is stored in the capacitor 64. A portion of the electrical powergenerated by the first motor MG1 is consumed in the driving of thesecond motor MG2.

The driving power of the second motor MG2 is outputted to the secondcarrier C2 along a path from the second motor gear Gm2 to the first ringouter periphery gear Gr1 to the first ring gear R1 to the H-clutch CH.At this time, the electrical power for driving the second motor MG2 issupplied from the first motor MG1 or from the capacitor 64 as needed.The driving power outputted to the second sun gear S2 as described aboveis outputted through the second planet gears P2 to the second ring gearR2, and the driving power outputted to the second carrier C2 isoutputted through the second planet gears P2 to the second ring gear R2.The driving power combined by the second ring gear R2 in this way istransmitted through the second ring outer periphery gear Gr2 and theoutput gear 71 to the output shaft 63.

The third motor MG3 is connected to either of the first motor MG1 or thesecond motor MG2 in the Hi-mode. When the third motor MG3 assists thefirst motor MG1 in the Hi-mode, the first motor clutch Cm1 is connectedand the second motor clutch Cm2 is disconnected in the same way as inthe Lo-mode. Therefore, the first connecting gear Ga1 is connected tothe rotating shaft Sm3 of the first motor MG1 and the second connectinggear Ga2 is disconnected from the rotating shaft Sm2 of the second motorMG2. As a result, the third motor MG3 is connected to the first motorMG1 via the third motor gear Gm3, the first connecting gear Ga1, and thefirst motor clutch Cm1. The third motor MG3 is disconnected from thesecond motor MG2 because the second motor clutch Cm2 is disconnected.

When the third motor MG3 assists the second motor MG2 in the Hi-mode,the first motor clutch Cm1 is disconnected and the second motor clutchCm2 is connected. Therefore, the second connecting gear Ga2 is connectedto the rotating shaft Sm2 of the second motor MG2 and the firstconnecting gear Ga1 is disconnected from the rotating shaft Sm1 of thefirst motor MG1. As a result, the third motor MG3 is connected to thesecond motor MG2 via the third motor gear Gm3, the first connecting gearGa1, the second connecting gear Ga2, and the second motor clutch Cm2.The third motor MG3 is disconnected from the first motor MG1 because thefirst motor clutch Cm1 is disconnected.

While forward travel driving has been discussed above, the operations ofreverse travel driving are the same. During braking, the roles of thefirst motor MG1 and the second motor MG2 as generator and motor arereversed from the above explanation.

The control of the power transmission device 24 by the control unit 27is described in detail below. The control unit 27 controls the outputtorque of the power transmission device 24 by controlling the motortorque of the first motor MG1, the second motor MG2, and the third motorMG3. That is, the control unit 27 controls the tractive force of thework vehicle 1 by controlling the motor torques of the first motor MG1,the second motor MG2, and the third motor MG3.

A method for determining the command torques to the first motor MG1 andthe second motor MG2 is explained below. FIG. 5 is a control blockdiagram illustrating processing executed by the control unit 27. Thecontrol unit 27 has a transmission requirement determination unit 84, anenergy management requirement determination unit 85, and a workimplement requirement determination unit 86 as illustrated in FIG. 5.

The transmission requirement determination unit 84 determines a requiredtractive force Tout on the basis of an accelerator operating amount Aacand an output rotation speed Nout. Specifically, the transmissionrequirement determination unit 84 determines the required tractive forceTout from the output rotation speed Nout on the basis of requiredtractive force characteristics information D1 stored in the storage unit56. The required tractive force characteristics information D1 is dataindicating the required tractive force characteristics for defining therelationship between the output rotation speed Nout and the requiredtractive force Tout. The required tractive force characteristics arechanged in response to the accelerator operating amount Aac. Therequired tractive force characteristics correspond to predeterminedvehicle speed-tractive force characteristics. The transmissionrequirement determination unit 84 uses the required tractive forcecharacteristics corresponding to the accelerator operating amount Aac todetermine the required tractive force Tout from the output rotationspeed Nout and to determine a transmission required horsepower Htm thatis a product of the output rotation speed Nout and the required tractiveforce Tout.

The energy management requirement determination unit 85 determines anenergy management required horsepower Hem on the basis of a remainingamount of electrical power in the capacitor 64. For example, the energymanagement required horsepower Hem is a horsepower required by the powertransmission device 24 for charging the capacitor 64. The energymanagement requirement determination unit 85 determines a currentcapacitor capacity from a voltage Vca of the capacitor 64. The energymanagement requirement determination unit 85 increases the energymanagement required horsepower Hem as the current capacitor capacitybecomes smaller.

The work implement requirement determination unit 86 determines the workimplement required horsepower Hpto on the basis of a work implement pumppressure Pwp and an operating amount Awo (referred to below as “workimplement operating amount Awo”) of the work implement operating member52 a. In the present exemplary embodiment, the work implement requiredhorsepower Hpto is a horsepower distributed to the work implement pump23. However, the work implement required horsepower Hpto may include ahorsepower distributed to the steering pump 30 and/or the transmissionpump 29. Specifically, the work implement requirement determination unit86 determines a required flow rate Qdm of the work implement pump 23from the work implement operating amount Awo on the basis of requiredflow rate information D2. The required flow rate information D2 isstored in the storage unit 56 and defines the relationship between therequired flow rate Qdm and the work implement operating amount Awo. Thework implement requirement determination unit 86 determines the workimplement required horsepower Hpto from the required flow rate Qdm andthe work implement pump pressure Pwp.

The control unit 27 has a target output shaft torque determining unit82, a target input shaft torque determining unit 81, and a motor commanddetermining unit 83.

The target output shaft torque determining unit 82 determines a targetoutput shaft torque To_ref. The target output shaft torque To_ref is atarget value for the torque to be outputted from the power transmissiondevice 24. The target output shaft torque determining unit 82 determinesthe target output shaft torque To_ref on the basis of the requiredtractive force Tout determined by the transmission requirementdetermination unit 84. Specifically, the target output shaft torqueTo_ref is determined by multiplying the required tractive force Tout bya predetermined distribution ratio. The predetermined distribution ratiois set, for example, so that the total of the work implement requiredhorsepower Hpto, the transmission required horsepower Htm, and theenergy management required horsepower Hem does not exceed the outputhorsepower from the engine 21.

The target input shaft torque determining unit 81 determines a targetinput shaft torque Te_ref. The target input shaft torque Te_ref is atarget value for the torque to be inputted to the power transmissiondevice 24. The target input shaft torque determining unit 81 determinesthe target input shaft torque Te_ref on the basis of the transmissionrequired horsepower Htm and the energy management required horsepowerHem. Specifically, the target input shaft torque determining unit 81calculates the target input shaft torque Te_ref by multiplying theengine rotation speed Ne by the sum of the energy management requiredhorsepower Hem and the value of the transmission required horsepower Htmmultiplied by the predetermined distribution ratio. The transmissionrequired horsepower Htm is calculated by multiplying the above-mentionedrequired tractive force Tout by the current output rotation speed Nout.

The motor command determining unit 83 uses torque-balance information todetermine command torques Tm1_ref and Tm2_ref for the respective motorsMG1 and MG2 from the target input shaft torque Te_ref and the targetoutput shaft torque To_ref. The torque-balance information defines arelationship between the target input shaft torque Te_ref and the targetoutput shaft torque To_ref so as to achieve a balance among the torquesof the power transmission device 24. The torque-balance information isstored in the storage unit 56.

As described above, the driving power drivetrains in the powertransmission device 24 are different for the Lo-mode and the Hi-mode. Asa result, the motor command determining unit 83 uses differenttorque-balance information to determine the command torques Tm1_ref andTm2_ref for the motors MG1 and MG2 in the Lo-mode and the Hi-mode.Specifically, the motor command determining unit 83 uses firsttorque-balance information represented by equation 1 below to determinecommand torques Tm1_Low and Tm2_Low for the motors MG1 and MG2 in the Lomode. In the present exemplary embodiment, the first torque-balanceinformation is an equation for balancing the torques of the powertransmission device 24.

Ts1_Low=Te_ref*r _(—) fr

Tc1_Low=Ts1_Low*(−1)*((Zr1/Zs1)+1)

Tr2_Low=To_ref*(Zod/Zo)

Ts2_Low=Tr2_Low*(Zs2/Zr2)

Tcp1_Low=Tc1_Low+Ts2_Low

Tm1_Low=Tcp1_Low*(−1)*(Zp1/Zp1d)

Tr1_Low=Ts1_Low*(Zr1/Zs1)

Tm2_Low=Tr1_Low*(−1)*(Zp2/Zp2d)  Equation 1

The motor command determining unit 83 uses second torque-balanceinformation represented by equation 2 below to determine command torquesTm1_Hi and Tm2_Hi for the motors MG1 and MG2 in the Hi-mode. In thepresent exemplary embodiment, the second torque-balance information isan equation for balancing the torques of the power transmission device24.

Ts1_Hi=Te_ref*r _(—) fr

Tc1_Hi=Ts1_Hi*(−1)*((Zr1/Zs1)+1)

Tr2_Hi=To_ref*(Zod/Zo)

Ts2_Hi=Tr2_Hi*(Zs2/Zr2)

Tcp1_Hi=Tc1_Hi+Ts2_Hi

Tm1_Hi=Tcp1_Hi*(−1)*(Zp1/Zp1d)

Tr1_Hi=Ts1_Hi*(Zr1/Zs1)

Tc2_Hi=Tr2_Hi*(−1)*((Zs2/Zr2)+1)

Tcp2_Hi=Tr1_Hi+Tc2_Hi

Tm2_Hi=Tcp2_Hi*(−1)*(Zp2/Zp2d)  Equation 2

The contents of the parameters in each torque-balance information aredepicted in Table 1 below.

TABLE 1 Te_ref Target input shaft torque To_ref Target output shafttorque r_fr Deceleration ratio for the FR switch mechanism 65 (The FRswitch mechanism 65 decelerates the engine rotation speed at l/r_fr tooutput. When the FR switch mechanism 65 is in the forward travel state,r_fr is a negative value. When the FR switch mechanism 65 is in thereverse travel state, r_fr is a positive value.) Zs1 Number of teeth ofthe sun gear S1 in the first planetary gear mechanism 68. Zr1 Number ofteeth of the ring gear R1 in the first planetary gear mechanism 68. Zp1Number of teeth in the first carrier gear Gc1 Zp1d Number of teeth ofthe first motor gear Gm1 Zs2 Number of teeth of the sun gear S2 in thesecond planetary gear mechanism 69. Zr2 Number of teeth of the ring gearR2 in the second planetary gear mechanism 69. Zp2 Number of teeth of thefirst ring outer periphery gear Gr1 Zp2d Number of teeth of the secondmotor gear Gm2 Zo Number of teeth of the second ring outer peripherygear Gr2 Zod Number of teeth of the output gear 71

The control of the engine 21 by the control unit 27 is described indetail below. As described above, the control unit 27 controls theengine 21 by transmitting command signals to the fuel injection device28. A method for determining the command throttle values for the fuelinjection device 28 will be explained below. The control unit 27 has anengine requirement determination unit 87 and a required throttledetermination unit 89.

The engine requirement determination unit 87 determines an enginerequired horsepower Hdm on the basis of the work implement requiredhorsepower Hpto, the transmission required horsepower Htm, and theenergy management required horsepower Hem. Specifically, the enginerequirement determination unit 87 determines the engine requiredhorsepower Hdm by adding the work implement required horsepower Hpto,the transmission required horsepower Htm, and the energy managementrequired horsepower Hem.

The required throttle determination unit 89 determines a commandthrottle value Th_cm from the engine required horsepower Hdm and theaccelerator operating amount Aac. The required throttle determinationunit 89 uses an engine torque line Let and a matching line Lma stored inthe storage unit 56 to determine the command throttle value Th_cm. Theengine torque line Let defines a relationship between the output torqueof the engine 21 and the engine rotation speed Ne. The matching line Lmais information for determining a first required throttle value from theengine required horsepower Hdm.

The required throttle determination unit 89 determines a first requiredthrottle value so that the engine torque line Let and the matching lineLma match at a matching point Pma1 where the output torque of the engine21 becomes the torque corresponding to the engine required horsepowerHdm. The required throttle determination unit 89 determines the lowestvalue from the first required throttle value and a second requiredthrottle value corresponding to the accelerator operating amount Aac asthe command throttle value Th_cm.

As described above, the third motor MG3 is switched between theconnected state and the disconnected state in the Lo mode. The thirdmotor MG3 is connected to either of the first motor MG1 or the secondmotor MG2 in the connected state. The third motor MG3 is disconnectedfrom the first motor MG1 and the second motor MG2 in the disconnectedstate. The control for the connection and disconnection of the thirdmotor MG3 is explained below.

Tm1_ref in the following explanation signifies Tm1_Low in the Lo-modeand Tm1_Hi in the Hi-mode. Tm2_ref in the following explanationsignifies Tm2_Low in the Lo-mode and Tm2_Hi in the Lo mode.

The magnitude correlation of the rotation speed and the torque in thefollowing explanation signifies a magnitude correlation of in absolutevalues. For example, although the rotation speed of the second motor MG2when the work vehicle 1 is traveling forward is negative when therotation speed of the first motor MG1 when the work vehicle 1 istraveling forward is positive, the magnitude correlation of the rotationspeed of the second motor MG2 signifies a magnitude correlation of theabsolute values of the rotation speed of the second motor MG2.

As illustrated in FIG. 6, the control unit 27 has a connectiondetermining unit 91, a motor switch control unit 92, and a predictedspeed computing unit 93. The connection determining unit 91 determineswhether assistance from the third motor MG3 is required or not. That is,the connection determining unit 91 determines whether to switch from thedisconnected state to the connected state of the third motor MG3 orwhether to switch from the connected state to the disconnected state ofthe third motor MG3.

First, processing when switching from the disconnected state to theconnected state of the third motor MG3 will be explained. FIG. 7 is aflow chart illustrating processing when the third motor MG3 is switchedfrom the disconnected state to the connected state.

As illustrated in FIG. 7, the connection determining unit 91 determinesin step S101 whether the first speed has been selected as the speedrange (first connection determination condition). The connectiondetermining unit 91 determines whether the first speed has been selecteddue to a detection signal from the speed change operation detecting unit53 b.

The connection determining unit 91 determines in step S102 whether thetractive force of the power transmission device 24 obtained from theoutput torque of the first motor MG1 and the output torque of the secondmotor MG2 is less than the required tractive force (second connectiondetermination condition). That is, the connection determining unit 91determines in step S102 that the tractive force obtained withoutassistance from the third motor MG3 is less than the required tractiveforce.

FIG. 8 is a flow chart illustrating a method for determining the secondconnection determination condition. As illustrated in step S201 in FIG.8, the connection determining unit 91 compares the command torqueTm1_ref for the first motor MG1 with a motor output limit valueTm1_limit and determines the smaller of the values as Tm1′. “Min” inFIG. 8 signifies the selection of the smallest value among the inputtednumerical values. That is, when Tm1_ref exceeds the motor output limitvalue Tm1_limit, the motor output limit value Tm1_limit is the upperlimit of Tm1′.

The connection determining unit 91 compares the command torque Tm2_reffor the second motor MG2 with a motor output limit value Tm2_limit anddetermines the smaller of the values as Tm2′ in step S202. The motoroutput limit values Tm1_limit and Tm1_limit are the maximum torques ofthe respective motors MG1 and MG2, for example. Alternatively, the motoroutput limit values Tm1_limit and Tm2_limit may be upper limit torquesdetermined in response to the rotation speeds of the motors MG1 and MG2.

In step S203, the connection determining unit 91 uses theabove-mentioned torque-balance information to determine aback-calculated target input shaft torque Te′ and a target output shafttorque To′ from the Tm1′ and Tm2′. The connection determining unit 91then compares in step S204 the target output shaft torque To_ref withthe back-calculated target output shaft torque To′ to determine whethera ratio r1 of To′ with respect to To_ref is less than a predeterminedthreshold Rth.

When the ratio r1 is less than the predetermined threshold Rth, theconnection determining unit 91 determines in step S205 that the tractiveforce is insufficient without the assistance from the third motor MG3.That is, the connection determining unit 91 determines that the secondconnection determination condition is met. When the ratio r1 is equal toor greater than the predetermined threshold Rth, the connectiondetermining unit 91 determines in step S206 that the tractive force issufficient without the assistance from the third motor MG3. That is, theconnection determining unit 91 determines that the second connectiondetermination condition is not met.

The threshold Rth is 100%. That is, when To′ is less than To_ref, theconnection determining unit 91 determines that the second connectiondetermination condition is met. However, the threshold Rth may be avalue less than 100%. In this case, although the generation of tractiveforce is delayed by the connection determination being met due to thetractive force being less than the required tractive force in comparisonto when the threshold Rth is 100%, hunting behavior of the connectiondetermination and disconnection determination can be suppressed becausehysteresis is provided between the connection determination and thebelow-mentioned disconnection determination.

As illustrated in FIG. 7, when either the first connection determinationcondition is met in step S101 or the second connection determinationcondition is met in step S102, the motor switch control unit 92 switchesthe third motor MG3 to the connected state in step S103. The motorswitch control unit 92 connects the third motor MG3 to whichever of thefirst motor MG1 and the second motor MG2 has the smallest rotationspeed. The following explanation discusses a case of connecting thethird motor MG3 to the first motor MG1.

When the first connection determination condition is not met in stepS101 and the second connection determination condition is not met instep S102, the motor switch control unit 92 maintains the third motorMG3 in the disconnected state in step S104. When the third motor MG3 isin the disconnected state, the motor command determining unit 83 setsthe command value of the rotation speed for the third motor MG3 to apredetermined first standby command value (this state is referred to asa “standby state” below). The predetermined first standby command valueis zero in the present exemplary embodiment. However, a very small valueother than zero may bet set as the predetermined first standby commandvalue. The very small value is preferably a torque for aiding a certainrotation to fall to zero rotations when the third motor MG3 is put onstandby at zero rotations. For example, the very small value is a torquefor preventing rotation of the third motor MG3 by canceling out aco-rotation torque that may be transmitted even during the clutchdisconnected state.

The motor command determining unit 83 synchronizes the third motor MG3when the motor switch control unit 92 switches the third motor MG3 fromthe disconnected state to the connected state. The motor commanddetermining unit 83 determines the command value for the third motor MG3so that the rotation speed of the third motor MG3 is synchronized withthe rotation speed of the motor to which the third motor MG3 isconnected among the first motor MG1 and the second motor MG2. That is,the motor command determining unit 83 determines the command torque forthe third motor MG3 so that the rotation speed of the third motor MG3approaches the rotation speed of the first motor MG1. The followingexplanation discusses synchronization control when connecting the thirdmotor MG3 to the first motor MG1 from the disconnected state.

FIG. 9 is a flow chart of processing for synchronization control of thethird motor MG3. As illustrated in FIG. 9, the predicted speed computingunit 93 detects a rotation speed of the first motor MG1 in step S301.The predicted speed computing unit 93 detects a rotation speed of thethird motor MG3 in step S302. The predicted speed computing unit 93detects the rotation speeds of the respective motors MG1 and MG3 on thebasis of signals from the inverters I1 and I3.

The predicted speed computing unit 93 computes a first predictedrotation speed in step S303. The first predicted rotation speed is apredicted value of the first rotation speed of the first motor MG1 aftera predetermined first predicted time period has elapsed from the currentpoint in time. The first rotation speed is the rotation speed of thefirst motor MG1 equivalent to the rotating shaft of the third motor MG3.A belowmentioned second rotation speed is the rotation speed of thesecond motor MG2 equivalent to the rotating shaft of the third motorMG3. The rotation speed equivalent to the rotating shaft of the thirdmotor MG3 signifies the rotation speed when the rotation speed of thefirst motor MG1 or the second motor MG2 is switched to the rotationspeed of the rotating shaft of the third motor MG3.

FIG. 10 illustrates changes between a first rotation speed, a firstpredicted rotation speed, and the rotation speed of the third motor MG3during synchronization control. The predicted speed computing unit 93records the rotation speed of the first motor MG1 at prescribed timeperiods and derives a rate of change of the rotation speed from therecorded rotation speeds. A solid line Lb_m1 in FIG. 10 depicts the rateof change of the first rotation speed derived from the recorded rotationspeeds of the first motor MG1. A dashed line Lb_m1′ depicts the changesof the first predicted rotation speed. The predicted speed computingunit 93 determines the first predicted rotation speed by calculating therotation speed after a first predicted time period Tth1 has elapsed fromthe rate of change of the first rotation speed. That is, a plurality ofrecent recording values among the recorded rotation speeds of the firstmotor MG1 are used to derive the rate of change of the first rotationspeed and the first predicted rotation speed is calculated as thecontinuation of the rate of change during the first predicted timeperiod Tth1. When using an average of the rate of change, the rate ofchange may also be calculated by adding a weight to the time sequence orby assuming that the amount of change of the rate of change continues. Asolid line Lb_m3 in FIG. 10 depicts the change in the rotation speed ofthe third motor MG3.

FIG. 10 further illustrates the connected (ON) states and thedisconnected (OFF) states of the first motor clutch Cm1. A solid lineLb_cm1 in FIG. 10 depicts changes in the command signals to the firstmotor clutch Cm1. A dashed line Lb_cm1′ depicts changes in the actualoil pressure of the first motor clutch Cm1.

As illustrated in FIG. 9, the connection determining unit 91 in stepS304 determines whether a difference dl between the first predictedrotation speed and the rotation speed of the third motor MG3 is equal toor less than a predetermined switching threshold Dth. When thedifference dl between the first predicted rotation speed and therotation speed of the third motor MG3 is equal to or less than thepredetermined switching threshold Dth, the motor switch control unit 92starts the connection with the first motor clutch Cm1 in step S305. Thatis, the motor switch control unit 92 outputs a command signal to theclutch control valve 38 to connect the first motor clutch Cm1.

When the connection with the first motor clutch Cm1 has started, thecounting of a timer Tm1 is started in step S306. The motor commanddetermining unit 83 sets the command torque for the third motor MG3 to apredetermined standby command value in step S307.

The connection determining unit 91 determines in step S308 whether apredetermined second predicted time period Tth2 has elapsed from theconnection starting time point with the first motor clutch Cm1. When thepredetermined second predicted time period Tth2 has not elapsed from theconnection starting time point with the first motor clutch Cm1, themotor command determining unit 83 keeps the command torque for the thirdmotor MG3 at the predetermined standby command value. That is, until thepredetermined second predicted time period Tth2 elapses from theconnection starting time point with the first motor clutch Cm1, themotor command determining unit 83 sets the command torque for the thirdmotor MG3 to the predetermined standby command value.

The standby command value in the present exemplary embodiment is zerobut may also be a value other than zero. The second predicted timeperiod Tth2 and the above-mentioned first predicted time period Tth1 arefor example estimated time periods from the connection starting timepoint of the first motor clutch Cm1 until the connection is completed.Alternatively, the second predicted time period Tth2 and the firstpredicted time period Tth1 may be time periods that take into accountthe time lag for the transmission of the detection signals of therotation speeds of the first motor MG1 and the third motor MG3 and thecommand signals for the first motor MG1 and the third motor MG3. Thesecond predicted time period Tth2 and the first predicted time periodTth1 are not necessarily the same value. For example, the secondpredicted time period Tth2 may be longer than the first predicted timeperiod Tth1.

When the second predicted time period Tth2 from the connection startingtime point of the first motor clutch Cm1 has elapsed, the motor commanddetermining unit 83 determines the command torques for the first motorMG1 and the third motor MG3 in step S309. That is, the torquedistribution to the third motor MG3 is carried out when it is determinedthat the connection of the third motor MG3 to the first motor MG1 iscompleted. The method for torque distribution is discussed below.

When the third motor MG3 switches from the standby state to theconnected state, the above synchronization control is not carried outwhen the rotation speed of the third motor MG3 and the rotation speed ofthe motor that is the connection target of the third motor MG3 are zero.For example, the rotation speed of the second motor MG2 is zero at theswitching point (threshold RSth1) between the Lo-mode and the Hi-mode.As a result, synchronization control is not carried out when the thirdmotor MG3 is connected to the second motor MG2 from the standby state ofa rotation speed of zero at the switching point between the Lo-mode andthe Hi-mode.

Next, processing when switching from the connected state to thedisconnected state of the third motor MG3 will be explained. FIG. 11 isa flow chart illustrating processing when the third motor MG3 isswitched from the connected state to the disconnected state.

The connection determining unit 91 determines in step S401 whether aspeed range other than the first speed has been selected (firstdisconnection determination condition). The connection determining unit91 determines whether a speed range other than the first speed has beenselected due to a detection signal from the speed change operationdetecting unit 53 b.

The connection determining unit 91 determines in S402 whether thetractive force of the power transmission device 24 obtained from theoutput torque of the first motor MG1 and the output torque of the secondmotor MG2 is sufficient with respect to the required tractive force(second disconnection determination condition). The connectiondetermining unit 91 at this time determines the second disconnectiondetermination condition with the same method as for the secondconnection determination condition described above with regard to FIG.8. However, Rth of the second disconnection determination condition maybe different from Rth of the connection determination condition. Rth ofthe disconnection determination condition is preferably 100%. A drop inthe tractive force at the time of disconnection can be prevented bymaking the Rth of the disconnection determination condition 100%.

When the first disconnection determination condition is met in step S401and the second disconnection determination condition is met in stepS402, the motor switch control unit 92 sets the third motor MG3 to thedisconnected state in step S403. When the third motor MG3 is in thedisconnected state, the motor command determining unit 83 sets the thirdmotor MG3 to the standby state.

The connection determining unit 91 determines in step S404 whether arotation speed RS3 of the third motor MG3 is larger than a predeterminedupper limit RS3_limit (third disconnection determination condition).When the rotation speed of the third motor MG3 is larger than thepredetermined upper limit RS3_limit, the motor switch control unit 92sets the third motor MG3 to the disconnected state in step S403. Thatis, either when either the first disconnection determination conditionis met in step S401 and the second disconnection determination conditionis met in step S402, or when a third disconnection determinationcondition is met in step S404, the motor switch control unit 92 sets thethird motor MG3 to the disconnected state.

When either the first disconnection determination condition in step S401or the second disconnection determination condition in step S402 is notmet, and the third disconnection determination condition is not met instep S404, the motor switch control unit 92 keeps the third motor MG3 inthe connected state.

As illustrated in FIG. 12, even when the above-mentioned connectiondetermination conditions are met, when the first rotation speed of thethird motor MG3 after being connected to the first motor MG1 exceeds apredetermined upper rotation speed RS3_limit′, the third motor MG3 maybe set to a second standby mode in the standby state. The third motorMG3 is set to the standby state in the second standby mode with thecommand value of the rotation speed of the third motor MG3 acting as asecond standby command value.

The second standby command value is equal to or lower than thepredetermined upper rotation speed RS3_limit′. The second standbycommand value may be larger than the above-mentioned first standbycommand value. The predetermined upper rotation speed RS3_limit′ ispreferably equal to or less than the above-mentioned predetermined upperlimit RS3_limit of the third disconnection determination condition. Thepredetermined upper rotation speed RS3_limit′ is more preferably lessthan the predetermined upper limit RS3_limit.

Even when the third motor MG3 is switched to the disconnected state whenthe first rotation speed exceeds the predetermined upper limit RS3_limitwhile the third motor MG3 is connected, the third motor MG3 is set tothe standby state at the second standby mode with the command value ofthe rotation speed of the third motor MG3 acting as the second standbycommand value while the connection determination condition continues tobe met.

In other words, the third motor MG3 is set to standby in the secondstandby mode when the driving power drivetrain is in the Lo-mode and thefirst speed is selected as the speed range while the third motor MG3 isin the disconnected state. As a result, the third motor MG3 can beconnected more quickly and the required tractive force can be generatedquickly when the first rotation speed falls from the high rotation speedduring excavation and the like.

As illustrated in FIG. 12, even when the above-mentioned condition forcausing the third motor MG3 to wait in the second standby mode is met,when the percentage of the speed ratio with respect to the thresholdRSth1 for switching between the Lo-mode and the Hi-mode is equal to orgreater than a predetermined percentage, the third motor MG3 may bet setto the standby state in the first standby mode. The command value of therotation speed of the third motor MG3 is set to the first standbycommand value in the first standby mode. That is, when the speed ratioin the Lo-mode approaches the threshold RSth1 for switching between theLo-mode and the Hi-mode and the speed ratio is equal to or greater thana predetermined ratio with respect to the threshold RSth1, the thirdmotor MG3 is switched from the second standby mode to the first standbymode. As a result, the command value of the rotation speed for the thirdmotor MG3 changes from the second standby command value to the firststandby command value. Consequently, the third motor MG3 can be made towait with the first standby command value in preparation to the Lo-modebeing switched to the Hi-mode.

As illustrated in FIG. 12, hysteresis may be provided during the switchbetween the first standby mode and the second standby mode. As a result,an increase in the load on the third motor MG3 and the third inverter I3due to frequent changing of the rotation speed in the standby state ofthe third motor MG3 can be suppressed. Alternatively, a transitionperiod in which the rotation speed in the standby state of the thirdmotor MG3 changes gradually in response to the rotation speed ratio maybe provided between the first standby mode and the second standby modein place of the provision of hysteresis.

Control of the connection and disconnection between the third motor MG3and the first motor MG1 has been discussed above, and the control of theconnection and disconnection between the third motor MG3 and the secondmotor MG2 is the same.

The third motor MG3 is always connected to either of the first motor MG1and the second motor MG2 in the Hi-mode. In this case, the connectiontarget of the third motor MG3 is switched between the first motor MG1and the second motor MG2. The switching control for the connectiontarget of the third motor MG3 is explained below.

As illustrated in FIG. 6, the control unit 27 has a switch determinationunit 94. The switch determination unit 94 determines the switching ofthe motor that is the connection target of the third motor MG3. Theswitch determination unit 94 connects the third motor MG3 to the motorhaving the lowest rotation speed among the first motor MG1 and thesecond motor MG2. Specifically, when the first rotation speed is lessthan the second rotation speed, the switch determination unit 94connects the third motor MG3 to the first motor MG1. When the secondrotation speed is less than the first rotation speed, the switchdetermination unit 94 connects the third motor MG3 to the second motorMG2.

FIG. 13 is a flow chart illustrating processing when the connectiontarget of the third motor MG3 is switched from the second motor MG2 tothe first motor MG1. As illustrated in FIG. 13, the predicted speedcomputing unit 93 detects the rotation speed of the first motor MG1 instep S501. The predicted speed computing unit 93 detects the rotationspeed of the second motor MG2 in step S502. The predicted speedcomputing unit 93 detects the rotation speeds of the respective motorsMG1 and MG2 on the basis of signals from the inverters I1 and I2. Therotation speeds of the motors MG1 and MG2 may be detected by signalsfrom a sensor for detecting the rotation speeds of the motors MG1 andMG2.

The predicted speed computing unit 93 computes a first predictedrotation speed RS_m1′ in step S503. The predicted speed computing unit93 computes a second predicted rotation speed RS_m2′ in step S504. Thefirst predicted rotation speed RS_m1′ is a predicted value of therotation speed of the first motor MG1 after the predetermined firstpredicted time period Tth1 has elapsed from the current point in time.The second predicted rotation speed RS_m2′ is a predicted value of therotation speed of the second motor MG2 after the predetermined firstpredicted time period Tth1 has elapsed from the current point in time.The predicted speed computing unit 93 computes the first predictedrotation speed RS_m1′ and the second predicted rotation speed RS_m2′using the same method as in step S303 for the above-mentionedsynchronization control.

FIG. 14 illustrates changes between the first rotation speed and secondrotation speed during switching control. A solid line Lc_m1 in FIG. 14depicts the rate of change of the first rotation speed derived from therecorded rotation speeds. A dashed line Lc_m1′ depicts the changes ofthe first predicted rotation speed RS_m1′. A solid line Lc_m2 depictsthe rate of change of the second rotation speed derived from therecorded rotation speeds. A dashed line Lc_m2′ depicts the changes ofthe second predicted rotation speed RS_m2′.

FIG. 14 further illustrates the connected (ON) states and thedisconnected (OFF) states of the first motor clutch Cm1 and the secondmotor clutch Cm2. The solid line Lc_cm1 in FIG. 14 depicts changes inthe command signals to the first motor clutch Cm1. The dashed lineLc_cm1′ depicts changes in the actual oil pressure of the first motorclutch Cm1. The solid line Lc_cm2 depicts changes in the command signalsto the second motor clutch Cm2. The dashed line Lc_cm2′ depicts changesin the actual oil pressure of the second motor clutch Cm2.

The switch determination unit 94 determines in step S505 in FIG. 13whether the first predicted rotation speed RS_m1′ has decreased from avalue larger than the second predicted rotation speed RS_m2′ to thesecond predicted rotation speed RS_m2′. When the dashed line Lc_m1′intersects with the dashed line Lc_m2′ in FIG. 14, it is determined thatthe first predicted rotation speed RS_m1′ has decreased from a valuelarger than the second predicted rotation speed RS_m2′ to the secondpredicted rotation speed RS_m2′.

When the first predicted rotation speed RS_m1′ decreases from a valuelarger than the second predicted rotation speed RS_m2′ to the secondpredicted rotation speed RS_m2′, the motor switch control unit 92controls the motor switching mechanism 73 to switch the connectiontarget of the third motor MG3 from the second motor MG2 to the firstmotor MG1 in step S506. That is, the motor switch control unit 92connects the first motor clutch Cm1 and disconnects the second motorclutch Cm2.

When the connection to the first motor clutch Cm1 has started, thecounting of a timer Tm2 is started in step S507. The motor commanddetermining unit 83 sets the command torque for the third motor MG3 tothe predetermined standby command value in step S508. As a result, thethird motor MG3 is set to the standby state. That is, the third motorMG3 is set to the standby state at the point in time that the connectionof the first motor clutch Cm1 and the disconnection of the second motorclutch Cm2 have started. The point in time is not limited to the pointin time that the connection of the first motor clutch Cm1 and thedisconnection of the second motor clutch Cm2 have started, and the thirdmotor MG3 may be switched to the standby state quickly thereafter.

Next, the switch determination unit 94 determines in step S509 whether apredetermined second predicted time period Tth2 has elapsed from theconnection starting time point with the first motor clutch Cm1. When thepredetermined second predicted time period Tth2 has not elapsed from theconnection starting time point with the first motor clutch Cm1, themotor command determining unit 83 keeps the command torque for the thirdmotor MG3 at the predetermined standby command value. That is, until thepredetermined second predicted time period Tth2 elapses from theconnection starting time point with the first motor clutch Cm1, themotor command determining unit 83 sets the command torque for the thirdmotor MG3 to the predetermined standby command value.

When the second predicted time period Tth2 from the connection startingtime point of the first motor clutch Cm1 has elapsed, the motor commanddetermining unit 83 determines the command torques for the first motorMG1 and the third motor MG3 in step S510. That is, the torquedistribution to the third motor MG3 is carried out when it is determinedthat the connection of the third motor MG3 to the first motor MG1 iscompleted. The method for torque distribution is discussed below.

While the control for switching the connection target of the third motorMG3 from the second motor MG2 to the first motor MG1 has been discussedabove, the control for switching the connection target of the thirdmotor MG3 from the first motor MG1 to the second motor MG2 is the same.That is, when the second predicted rotation speed RS_m2′ decreases froma value larger than the first predicted rotation speed RS_m1′ to thefirst predicted rotation speed RS_m1′, the motor switch control unit 92controls the motor switching mechanism 73 to switch the connectiontarget of the third motor MG3 from the first motor MG1 to the secondmotor MG2. The motor command determining unit 83 sets the command torquefor the third motor MG3 to the predetermined standby command value fromthe point in time that the second predicted rotation speed reaches thefirst predicted rotation speed until a third predicted time period haselapsed. The third predicted time period in the present exemplaryembodiment may be the same value as the second predicted time period, ormay be a different value.

Next, the processing for torque distribution to the third motor MG3 isdiscussed.

When the third motor MG3 is connected to the first motor MG1, the motorcommand determining unit 83 distributes a portion of the abovementionedTm1_ref to the third motor MG3 while Tm1_ref is considered as a requiredtorque for the third motor MG3 and the first motor MG1. When the thirdmotor MG3 is connected to the second motor MG2, the motor commanddetermining unit 83 distributes a portion of the above-mentioned Tm2_refto the third motor MG3 while Tm2_ref is considered as a required torquefor the third motor MG3 and the second motor MG2. Hereinbelow,processing when the third motor MG3 is connected to the first motor MG1and Tm1_ref is distributed to the first motor MG1 and the third motorMG3 will be discussed.

FIG. 15 illustrates command torques for the motors with respect to therequired torque. A solid line Ld_m1 in FIG. 15 depicts the commandtorque for the first motor MG1 with respect to the required torque. Adashed line Ld_m3 depicts the command torque for the third motor MG3with respect to the required torque. As illustrated in FIG. 15, themotor command determining unit 83 determines the command torques for thefirst motor MG1 and the third motor MG3 so that the command torque forthe third motor MG3 and the command torque for the first motor MG1 arethe same. While the solid line Ld_m1 and the dashed line Ld_m3 aredepicted as slightly away from each other for ease of understanding inFIG. 15, the solid line Ld_m1 and the dashed line Ld_m3 match each otherprecisely.

In this case, the command torque Tm1 to the first motor MG1 and thecommand torque Tm3 to the third motor MG3 are expressed as in thefollowing equation 3.

Tm1=Tm3=Tm1_ref/(1+r)  Equation 3

Here, r is a speed increasing ratio of the third motor MG3 with respectto the first motor MG1. That is, the rotation speed of the third motorMG3 is r times the rotation speed of the first motor MG1. The torque ofthe third motor MG3 is multiplied by r and transmitted to the rotatingshaft of the first motor MG1. When r is greater than one, the torque ofthe third motor MG3 is multiplied by r and assists the first motor MG1so that the size of the motors can be further reduced.

While the processing for distributing Tm1_ref to the first motor MG1 andthe third motor MG3 has been explained above, the processing todistribute the Tm2_ref to the second motor MG2 and the third motor MG3is the same.

FIG. 16 illustrates changes in the rotation speeds of the first motorMG1, the second motor MG2, and the third motor MG3 with respect to therotation speed ratio in the present embodiment. A long dashed shortdashed line La_m1 in FIG. 16 depicts the rotation speed of the firstmotor MG1, and a long dashed short dashed line La_m2 depicts therotation speed of the second motor MG2 in the same way as in FIG. 4. Asolid line La_m3 depicts the rotation speed of the third motor MG3. Thethird motor MG3 is only connected to the first motor MG1 in the Lo-modeand is not connected to the second motor MG2 and is set to thedisconnected state even when the rotation speed of the second motor MG2is less than the first motor MG1.

FIG. 16 further illustrates changes in the command torques to the motorsMG1, MG2, and MG3 when the above-mentioned torque distribution iscarried out. A long dashed short dashed line Le_m1 in FIG. 16 depictsthe command torque of the first motor MG1, and a long dashed shortdashed line Le_m2 depicts the command torque of the second motor MG2. Asolid line Le_m3 depicts the command torque of the third motor MG3. FIG.16 illustrates changes of the connected (ON) states and the disconnected(OFF) states of the first motor clutch Cm1 and the second motor clutchCm2. A solid line La_cm1 in FIG. 16 depicts changes in the commandsignals to the first motor clutch Cm1 in the same way as in FIG. 4. Asolid line La_cm2 depicts changes in the command signals to the secondmotor clutch Cm2.

FIG. 17 illustrates changes in rotation speeds of the first motor MG1and the second motor MG2 with respect to the vehicle speed ratio in acomparative example. Assistance by the third motor MG3 is not carriedout in the comparative example. A long dashed short dashed line La_m1′in FIG. 17 depicts the rotation speed of the first motor MG1 in thecomparative example, and a long dashed short dashed line La_m2′ depictsthe rotation speed of the second motor MG2 in the comparative example.FIG. 17 further illustrates changes in the command torques for themotors MG1 and MG2 in the comparative example. A long dashed shortdashed line Le_m1′ in FIG. 17 depicts the command torque of the firstmotor MG1, and a long dashed short dashed line Le_m2′ depicts thecommand torque of the second motor MG2.

As illustrated in FIG. 17, the torques of the motors increase when therotation speeds of the motors decrease. As a result, when the rotationspeed of the first motor MG1 becomes a small value approaching zero inthe comparative example as illustrated in FIG. 17 (region A), thecommand torque for the first motor MG1 exceeds a boundary torque Tlim1.When the rotation speed of the second motor MG2 becomes a small valueapproaching zero (region B), the command torque for the second motor MG2exceeds a boundary torque Tlim2.

Conversely as illustrated in FIG. 16, when the rotation speed of thefirst motor MG1 is less than the rotation speed of the second motor MG2in the Lo-mode in the present embodiment (region A1), the third motorMG3 is connected to the first motor MG1 and the required torque isevenly distributed between the first motor MG1 and the third motor MG3.As a result, the command torque for the first motor MG1 and the commandtorque for the third motor MG3 are set so as not to exceed the boundarytorque Tlim1.

Even when the rotation speed of the second motor MG2 is less than therotation speed of the first motor MG1 in the Lo-mode (region A2), thethird motor MG3 is not connected to the second motor MG2 and is set tothe standby state. However, even in this case, the required torque forthe second motor MG2 does not exceed the boundary torque Tlim2 and thusthe command torque for the second motor MG2 does not exceed the boundarytorque Tlim2.

When the rotation speed of the second motor MG2 is less than therotation speed of the first motor MG1 in the Hi-mode (region B1), thethird motor MG3 is connected to the second motor MG2 and the requiredtorque is evenly distributed between the second motor MG2 and the thirdmotor MG3. As a result, the command torque for the second motor MG2 andthe command torque for the third motor MG3 are set so as not to exceedthe boundary torque Tlim2.

When the rotation speed of the first motor MG1 is less than the rotationspeed of the second motor MG2 in the Hi-mode (region B2), the thirdmotor MG3 is connected to the first motor MG1 and the required torque isevenly distributed between the first motor MG1 and the third motor MG3.As a result, the command torque for the first motor MG1 and the commandtorque for the third motor MG3 are set so as not to exceed the boundarytorque Tlim1.

The work vehicle according to the present exemplary embodiment has thefollowing features.

When assistance from the third motor is not required, the disconnectedstate in which the third motor MG3 is disconnected from the first motorMG1 and the second motor MG2 is set. As a result, the occurrence of lossin the third motor MG3 can be suppressed and the power transmissionefficiency of the power transmission device 24 can be improved.

When the third motor MG3 is in the disconnected state, the motor commanddetermining unit 83 sets the command value for the rotation speed of thethird motor MG3 to the predetermined standby command value. As a result,the generation of loss in the third motor MG3 can be further suppressed.When the third motor MG3 is set to the disconnected state, the commandvalue for the torque and not the rotation speed for the third motor MG3may be set to the predetermined standby command value.

When the third motor MG3 is switched from the disconnected state to thefirst connected state, the motor command determining unit 83 determinesthe command value for the third motor so that the rotation speed of thethird motor is synchronized with the rotation speed of the first motorMG1. As a result, the third motor MG3 and the first motor MG1 can beconnected smoothly. When the third motor is switched from thedisconnected state to the second connected state, the motor commanddetermining unit 83 determines the command value for the third motor MG3so that the rotation speed of the third motor MG3 is synchronized withthe rotation speed of the second motor MG2. As a result, the occurrenceof a shock during connection can be suppressed.

When the connection target of the third motor MG3 is switched from thesecond motor MG2 to the first motor MG1, the motor switch control unit92 starts the connection with the first motor clutch Cm1 when thedifference between the first predicted rotation speed and the rotationspeed of the third motor MG3 is equal to or less than the predeterminedswitching threshold. The motor switch control unit 92 disconnects thesecond motor clutch Cm2. As a result, the third motor MG3 can beswitched from the disconnected state to the first connected state at atiming when the first rotation speed approaches the rotation speed ofthe third motor MG3 in consideration of the time required for switchingthe first motor clutch Cm1. Similarly, when the connection target of thethird motor MG3 is switched from the first motor MG1 to the second motorMG2, the motor switch control unit 92 starts the connection with thesecond motor clutch Cm2 when the difference between the second predictedrotation speed and the rotation speed of the third motor MG3 is equal toor less than the predetermined switching threshold. The motor switchcontrol unit 92 disconnects the first motor clutch Cm1. As a result, thethird motor MG3 can be switched from the disconnected state to thesecond connected state at a timing when the second rotation speedapproaches the rotation speed of the third motor MG3 in consideration ofthe time required for switching the second motor clutch Cm2. As aresult, the occurrence of a shock during connection can be suppressed.The order for connecting and disconnecting the motor clutches Cm1 andCm2 may be set so that either of the connecting or the disconnecting iscarried out first or the connecting and the disconnecting may be set tooccur at the same time.

Until the predetermined second predicted time period Tth2 elapses fromthe connection starting time point with the first motor clutch Cm1 orthe second motor clutch Cm2, the motor command determining unit 83 setsthe command torque for the third motor MG3 to the predetermined standbycommand value. As a result, the command torque for the third motor MG3becomes the predetermined standby command value until the connection ofthe first motor clutch Cm1 or the second motor clutch Cm2 is complete.As a result, when the rotation speed of the third motor MG3 fluctuatesdue to torque transmission when connecting the clutches, the third motorMG3 can be prevented from outputting an unnecessary torque during therecovery of the rotation speed. As a result, the occurrence of a shockduring connection can be further suppressed.

When the second predicted time period from the connection starting timepoint of the first motor clutch Cm1 has elapsed when the third motor MG3is connected to the first motor MG1, the motor command determining unit83 determines the command torques for the first motor MG1 and for thethird motor MG3 on the basis of the required command torque. As aresult, the assistance from the third motor MG3 can be started at atiming approaching the completion time point of the connection with thefirst motor clutch Cm1. When the second predicted time period from theconnection starting time point of the second motor clutch Cm2 haselapsed when the third motor MG3 is connected to the second motor MG2,the motor command determining unit 83 determines the command torques forthe second motor MG2 and for the third motor MG3 on the basis of therequired command torque. As a result, the assistance from the thirdmotor MG3 can be started at a timing approaching the completion timepoint of the connection with the second motor clutch Cm2.

The first connection determination condition for determining whetherassistance from the third motor MG3 is required or not is the selectionof the speed range of the first speed by the speed change operatingmember 53 a. In this case, the third motor MG3 is set to the connectedstate when the first speed in the speed range is selected when theoperator requires a large torque. As a result, a large torque can beobtained due to the third motor MG3 assisting the first motor MG1 or thesecond motor MG2.

The second connection determination condition for determining whetherthe assistance from the third motor MG3 is necessary is based on whetherthe tractive force of the power transmission device 24 obtained from theoutput torque of the first motor MG1 and the output torque of the secondmotor MG2 with regard to the required tractive force is lack. As aresult, the third motor MG3 is set to the connected state when theassistance from the third motor MG3 is required for obtaining therequired tractive force. As a result, the required tractive force can beobtained due to the third motor MG3 assisting the first motor MG1 or thesecond motor MG2.

Although an exemplary embodiment of the present invention has beendescribed so far, the present invention is not limited to the aboveembodiments and various modifications may be made within the scope ofthe invention.

The present invention may be applicable to another type of speed changedevice such as a HMT without being limited to the EMT. In this case, thefirst motor MG1, the second motor MG2, and the third motor MG3 functionas hydraulic motors and hydraulic pumps. The first motor MG1, the secondmotor MG2, and the third motor MG3 are variable capacitor pump/motors,and the capacities are controlled by the control unit 27.

The configuration of the power transmission device 24 is not limited tothe configuration of the above exemplary embodiments. For example, thecoupling and disposition of the elements of the two planetary gearmechanisms 68 and 69 are not limited to the coupling and disposition ofthe above exemplary embodiments. The number of planetary gear mechanismsis not limited to two. For example, the power transmission device 24 mayonly have one planetary gear mechanism.

While the switching of the connection target of the third motor MG3 isdetermined based on the first predicted rotation speed and the secondpredicted rotation speed in the above exemplary embodiment, theswitching of the connection target of the third motor MG3 may bedetermined based on the first rotation speed and the second rotationspeed.

The third motor MG3 may be connected to the second motor MG2 in theLo-mode. The third motor MG3 may be constantly connected to either ofthe first motor MG1 or the second motor MG2 in the Lo-mode.

While the connection determining unit 91 and the switch determinationunit 94 estimate the timing when the connection of the first motorclutch Cm1 or the second motor clutch Cm2 is finished based on thesecond predicted time period Tth2 in the above exemplary embodiment, thecompletion of the connection of the first motor clutch Cm1 or the secondmotor clutch Cm2 may be determined based on a detection signal from aclutch replenishment detecting unit 39 illustrated in FIG. 18. Theclutch replenishment detecting unit 39 detects when the replenishment ofhydraulic fluid discharged from the clutch control valve 38 to the firstmotor clutch Cm1 and the second motor clutch Cm2 is completed. Theclutch replenishment detecting unit 39 is, for example, a switch thatreacts to an action of a clutch plate provided in the motor clutches Cm1or Cm2. Alternatively, the clutch replenishment detecting unit 39 may bea pressure sensor or a pressure switch for detecting the oil pressure ofthe motor clutches Cm1 and Cm2.

In this case, the motor command determining unit 83 sets the commandtorque for the third motor MG3 to the predetermined standby commandvalue from the connection starting time point with the first motorclutch Cm1 or the second motor clutch Cm2 until the detection by theclutch replenishment detecting unit 39 detecting the completion of thereplenishment. When the clutch replenishment detecting unit 39 detectsthat the replenishment is complete after the start of the connection tothe first motor clutch Cm1 or the second motor clutch Cm2, the motorcommand determining unit 83 determines the command torques for the firstmotor MG1 or the second motor MG2 and for the third motor MG3 on thebasis of the required command torque.

The command torque for the third motor MG3 is set so as to have the samecommand torque as the motor that is the connection target of the thirdmotor MG3, for example, the first motor MG1, during the torquedistribution to the third motor MG3 in the above embodiment. However,the motor command determining unit 83 may determine the command torquesto the first motor MG1 and the third motor MG3 so that the commandtorque for the third motor MG3 is equal to or less than the commandtorque for the first motor MG1, and the torque distribution to the thirdmotor MG3 is not limited to the method of the above exemplaryembodiment.

For example, FIG. 19 illustrates a torque distribution to the thirdmotor MG3 according to a first modified example. FIG. 19 illustrates acommand torque for the first motor MG1 and a command torque for thethird motor MG3 with respect to the required torque when the third motorMG3 is connected to the first motor MG1. A dashed line Lf_m1 in FIG. 19depicts the command torque for the first motor MG1 with respect to therequired torque. A solid line Lf_m3 depicts the command torque for thethird motor MG3 with respect to the required torque.

As illustrated in FIG. 19, when the required torque is equal to or lessthan the predetermined boundary torque Tlim1, the motor commanddetermining unit 83 determines the required torque as the command torquefor the first motor MG1 and sets the command torque for the third motorMG3 to the predetermined standby command value (zero in the presentmodified example). When the required torque is greater than thepredetermined boundary torque Tlim1, the boundary torque Tlim1 isdetermined as the command torque of the first motor MG1 and thedifference between the required torque and the boundary torque Tlim1 isdetermined as the command torque of the third motor MG3. In this case,the command torque Tm1 for the first motor MG1 and the command torqueTm3 for the third motor MG3 are expressed as in the following equation4.

When Tm1_ref≦Tlim1:

Tm1=Tm1_ref

Tm3=0

When Tm1_ref>Tlim1:

Tm1=Tlim1

Tm3=(Tm1_ref−Tlim1)/r  Equation 4

When the required torque is greater than the predetermined boundarytorque Tlim1 in the torque distribution method for the third motor MG3according to the first modified example, the first motor MG1 can beassisted by the third motor MG3. When the required torque is equal to orless than the predetermined boundary torque Tlim1, the generation ofshock can be suppressed even when the connection with the third motorMG3 is switched because the predetermined standby command value is thecommand torque of the third motor MG3. When the third motor MG3 isconnected to the second motor MG2, the command torque for the secondmotor MG2 is determined in the same way as the command torque for thefirst motor MG1 described above.

FIG. 20 illustrates torque distribution for the third motor MG3according to a second modified example. FIG. 20 illustrates a commandtorque for the first motor MG1 and a command torque for the third motorMG3 with respect to the required torque when the third motor MG3 isconnected to the first motor MG1. A dashed line Lg_m1 in FIG. 20 depictsthe command torque for the first motor MG1 with respect to the requiredtorque. A solid line Lg_m3 depicts the command torque for the thirdmotor MG3 with respect to the required torque.

As illustrated in FIG. 20, when the required torque is equal to or lessthan a predetermined torque threshold Tqth1, the motor commanddetermining unit 83 determines the required torque as the command torquefor the first motor MG1 and sets the command torque for the third motorMG3 to the predetermined standby command value (zero in the presentmodified example). When the required torque is greater than thepredetermined torque threshold Tqth1 and the command torque of the thirdmotor MG3 is equal to or less than the torque threshold Tqth1, thetorque threshold Tqth1 is determined as the command torque for the firstmotor MG1 and the difference between the required torque and the torquethreshold Tqth1 is determined as the command torque of the third motorMG3. When the required torque is greater than the predetermined torquethreshold Tqth1 and the command torque of the third motor MG3 is greaterthan the torque threshold Tqth1, the command torques for the first motorMG1 and the third motor MG3 are determined so that the command torque ofthe first motor MG1 and the command torque of the third motor MG3 arethe same. In this case, the command torque Tm1 for the first motor MG1and the command torque Tm3 for the third motor MG3 are expressed as inthe following equation 5.

When Tm1_ref≦Tlim1:

Tm1=Tm1_ref

Tm3=0

When Tlim1<Tm1_ref≦Tlim1*(1+r):

Tm1=Tlim1

Tm3=(Tm1_ref−Tlim1)/r

When Tlim1*(1+r)<Tm1_ref:

Tm1=Tm3=Tm1_ref/(1+r)  Equation 5

When the required torque is equal to or less than the predeterminedtorque threshold Tqth1 in the torque distribution method for the thirdmotor MG3 according to the second modified example, the generation ofshock can be suppressed even when the connection with the third motorMG3 is switched because the command torque of the third motor MG3 is thepredetermined standby command value. When the required torque is greaterthan the torque threshold Tqth1 and the command torque of the thirdmotor MG3 is equal to or less than the torque threshold Tqth1, the firstmotor MG1 can be assisted by the third motor MG3. When the requiredtorque is greater than the predetermined torque threshold Tqth1 and thecommand torque of the third motor MG3 is greater than the torquethreshold Tqth1, the assisted torque from the third motor MG3 increasesbecause the third motor MG3 outputs a torque that is the same size asthe first motor MG1. As a result, the output torque of the first motorMG1 can be reduced and wear on the components for transmitting theoutput torque of the first motor MG1 in the power transmission device 24can be suppressed. When the third motor MG3 is connected to the secondmotor MG2, the command torque for the second motor MG2 is determined inthe same way as the command torque for the first motor MG1 describedabove.

The method for determining the second connection determination conditionmay be changed without being limited to the method for determination ofthe above embodiment. For example, when rm1 is equal to or greater thanRm1 th (rm1≧Rm1 th), the connection determining unit 91 may determinethat the second connection determination condition with regard to thefirst motor MG1 is met. Thus, rm1=(Tm1_ref/Tm1_limit). Therefore, whenTm1_ref is equal to or greater than the predetermined upper limitthreshold (Rm1 th*Tm1_limit), the connection determining unit 91 maydetermine that the second connection determination condition with regardto the first motor MG1 is met.

Similarly, when rm2 is equal to or greater than Rm2 th (rm2≧Rm2 th), theconnection determining unit 91 may determine that the second connectiondetermination condition with regard to the second motor MG2 is met. Whenthe second connection determination condition with regard to the firstmotor MG1 and the second connection determination condition with regardto the second motor MG2 are met at the same time, the abovementionedfirst rotation speed and the second rotation speed are compared and thethird motor MG3 may be connected to the motor corresponding to thelowest rotation speed.

For example, Rm1 th is equal to 1 (Rm1 th=1). In this case, the requiredtractive force can be generated. Alternatively, Rm1 th may be a valueless than one. In this case, the second connection determinationcondition is met when the required tractive force approaches the limitvalue. As a result, the required tractive force can be generated morequickly because synchronization and the connection of the third motorMG3 can be started before the required tractive force reaches the limitvalue before the. Alternatively, Rm1 th may be a value greater than one.In this case, while the required tractive force may not necessarily begenerated, the connection frequency of the third motor MG3 is reducedwhereby wear and degradation of the mechanical elements such as gearteeth and electrical elements such as the inverters can be suppressed.

Instead of the above-mentioned method for determining the seconddisconnection determination condition, when rm1 is less than Rm1 th(rm1<Rm1 th), the connection determining unit 91 may determine that thesecond disconnection determination condition with regard to the firstmotor MG1 is met. That is, when Tm1_ref is less than the predeterminedupper limit threshold (Rm1 th*Tm1_limit), the connection determiningunit 91 may determine that the second disconnection determinationcondition with regard to the first motor MG1 is met. In this case, Rmth1of the disconnection determination condition, is preferably equal to orless than the Rm1 th of the connection determination condition. Morepreferably, Rmth1 of the disconnection determination condition is lessthan the Rm1 th of the connection determination condition. In this case,hunting behavior during the switching between the connection anddisconnection can be suppressed and the switching can be stabilizedbecause hysteresis is provided between the disconnection determinationcondition and the connection determination condition.

According to exemplary embodiments of the present invention, ahybrid-type work vehicle in which the power transmission efficiency ofthe power transmission device is improved and a control method of thework vehicle can be provided.

1. A work vehicle comprising: an engine; a hydraulic pump driven by theengine; a work implement driven by hydraulic fluid discharged from thehydraulic pump; a travel device driven by the engine; a powertransmission device that transmits driving power from the engine to thetravel device; and a control unit for controlling the power transmissiondevice; the power transmission device including an input shaft; anoutput shaft; a gear mechanism that has a planetary gear mechanism andthat transmits rotation of the input shaft to the output shaft; and afirst motor connected to first rotating elements of the planetary gearmechanism; a second motor connected to second rotating elements of theplanetary gear mechanism; a third motor for assisting the first motorand the second motor; and a motor switching mechanism configured toselectively switch between a connected state in which the third motor isconnected to the first motor or the second motor and a disconnectedstate in which the third motor is disconnected from both the first motorand the second motor; and the power transmission device is beingconfigured to change the rotation speed ratio of the output shaft withrespect to the input shaft by changing the rotation speeds of the firstmotor, the second motor, and the third motor, and the control unitincluding a connection determining unit that determines whetherassistance from the third motor is required or not; and a motor switchcontrol unit for controlling the motor switching mechanism so that whenthe connection determining unit determines that assistance from thethird motor is required, the third motor is set to the connected state,and when the connection determining unit determines that assistance fromthe third motor is not required, the third motor is set to thedisconnected state.
 2. The work vehicle according to claim 1, whereinthe control unit further includes a motor command value determining unitfor determining a command value for a torque or for a rotation speed ofthe third motor; and when the third motor is in the disconnected state,the motor command value determining unit sets the command value for thetorque or the rotation speed of the third motor to a predetermined firststandby command value.
 3. The work vehicle according to claim 2, whereinwhen the third motor is switched from the disconnected state to theconnected state, the motor command value determining unit determines thecommand value for the third motor so that the rotation speed of thethird motor is synchronized with the rotation speed of the motor to beconnected with the third motor among the first motor and the secondmotor.
 4. The work vehicle according to claim 3, wherein the motorswitching mechanism has a clutch for switching between connection anddisconnection of the third motor with the first motor or the secondmotor; the third motor is switched from the disconnected state to theconnected state by switching the clutch from the disconnected state tothe connected state; the control unit has a predicted speed computingunit that computes a predicted rotation speed that is a predicted valueof the rotation speed of the first motor or the second motor after apredetermined first predicted time period has elapsed from a currentpoint in time; and when a difference between a rotation speed of thepredicted rotation speed corresponding to a rotating shaft of the thirdmotor and the rotation speed of the third motor is equal to or less thana predetermined switching threshold, the motor switch control unitstarts the connection of the clutch.
 5. The work vehicle according toclaim 4, wherein the motor command value determining unit sets a commandtorque for the third motor to a predetermined standby command valueuntil a predetermined second predicted time period from a connectionstarting time point of the clutch has elapsed.
 6. The work vehicleaccording to claim 5, wherein the motor command value determining unitdetermines a required command torque for the motor connected with thethird motor; and when the second predicted time period from theconnection starting time point of the clutch has elapsed, the motorcommand value determining unit determines the command torques for thethird motor and the motor connected with the third motor on the basis ofthe required command torque.
 7. The work vehicle according to claim 4,further comprising a clutch replenishment detecting unit for detectingwhen a replenishment of hydraulic fluid to the clutch is completed; themotor command value determining unit setting the command torque for thethird motor to a predetermined standby command value until the clutchreplenishment detecting unit detects that the replenishment is completedfrom the connection starting time point of the clutch.
 8. The workvehicle according to claim 7, wherein the motor command valuedetermining unit determines a required command torque for the motorconnected with the third motor; and when the clutch replenishmentdetecting unit detects that the replenishment is completed after thestart of the connection of the clutch, the motor command valuedetermining unit determines the command torque for the third motor andthe motor connected with the third motor on the basis of the requiredcommand torque.
 9. The work vehicle according to claim 1, furthercomprising a speed change operating member for selecting a speed rangethat defines an upper limit of a vehicle speed; the connectiondetermining unit determining that the assistance from the third motor isrequired when a predetermined connection determination condition is met,and the predetermined connection determination condition including aselection of the lowest speed range by the speed change operatingmember.
 10. The work vehicle according to claim 1, further comprising avehicle speed detecting unit for detecting a vehicle speed; and anaccelerator operating member; the control unit further including atransmission requirement determination unit for determining a requiredtractive force of the power transmission device on the basis of thevehicle speed and the operating amount of the accelerator operatingmember; the connection determining unit determining that the assistancefrom the third motor is required when a predetermined connectiondetermination condition is met, and the predetermined connectiondetermination condition including a tractive force obtained from anoutput torque of the first motor and an output torque of the secondmotor being less than the required tractive force.
 11. The work vehicleaccording to claim 1, further comprising a speed change operating memberfor selecting a speed range that defines an upper limit of a vehiclespeed; a vehicle speed detecting unit for detecting a vehicle speed; andan accelerator operating member; the control unit further including atransmission requirement determination unit for determining a requiredtractive force of the power transmission device on the basis of thevehicle speed and the operating amount of the accelerator operatingmember; the connection determining unit determining that the assistancefrom the third motor is not required when a predetermined disconnectiondetermination condition is met, and the predetermined disconnectiondetermination condition including a selection of a speed range otherthan the lowest speed range by the speed change operating member, and atractive force obtained from an output torque of the first motor and anoutput torque of the second motor not being less than the requiredtractive force.
 12. The work vehicle according to claim 1, wherein themotor switch control unit controls the motor switching mechanism so thatthe third motor is set to the disconnected state when the rotation speedof the third motor is greater than a predetermined upper limit value.13. The work vehicle according to claim 2, wherein when the rotationspeed of the third motor after the third motor is set to the connectedstate exceeds a predetermined upper limit rotation speed, the motorswitch control unit switches the third motor to the disconnected stateand the motor command value determining unit sets the command value ofthe rotation speed of the third motor to a predetermined second standbycommand value which is greater than the first standby command value. 14.The work vehicle according to claim 10, wherein the connectiondetermining unit determines that a tractive force obtained from theoutput torque of the first motor and the output torque of the secondmotor is less than the required tractive force when the output torque ofthe first motor or the output torque of the second motor which arerequired for generating the required tractive force are equal to orgreater than a predetermined upper limit threshold.
 15. The work vehicleaccording to claim 1, wherein the motor switching mechanism has a clutchfor switching between connection or disconnection of the first motor orthe second motor with the third motor; and the third motor is switchedfrom the disconnected state to the connected state by switching theclutch from the disconnected state to the connected state; the controlunit including a motor command value determining unit for determining acommand value of a torque or a rotation speed for the third motor and arequired command torque for the motor connected to the third motor; apredicted speed computing unit that computes a predicted rotation speedthat is a predicted value of the rotation speed of the first motor orthe second motor after a predetermined first predicted time period haselapsed from a current point in time; when the third motor is in thedisconnected state, the motor command value determining unit sets thecommand value for the torque or the rotation speed of the third motor toa predetermined first standby command value; when the third motor isswitched from the disconnected state to the connected state, the motorcommand value determining unit determines the command value for thethird motor so that the rotation speed of the third motor issynchronized with the rotation speed of the motor connected with thethird motor among the first motor and the second motor; when adifference between a rotation speed of the predicted rotation speedcorresponding to a rotating shaft of the third motor and the rotationspeed of the third motor is equal to or less than a switching threshold,the motor switch control unit starts the connection of the clutch; untila predetermined second predicted time period has elapsed from aconnection starting time point of the clutch, the motor command valuedetermining unit sets the command torque for the third motor to apredetermined standby command value; when the second predicted timeperiod from the connection starting time point of the clutch haselapsed, the motor command value determining unit determines the commandtorque for the third motor and the motor connected with the third motoron the basis of the required command torque; and when the rotation speedof the third motor after the third motor is set to the connected stateexceeds a predetermined upper limit rotation speed, the motor switchcontrol unit switches the third motor to the disconnected state and themotor command value determining unit sets the command value of therotation speed of the third motor to a predetermined second standbycommand value which is greater than the first standby command value. 16.The work vehicle according to claim 1 further comprising a speed changeoperating member for selecting a speed range that defines an upper limitof a vehicle speed; a vehicle speed detecting unit for detecting avehicle speed; and an accelerator operating member; the control unitfurther including a transmission requirement determination unit fordetermining a required tractive force of the power transmission deviceon the basis of the vehicle speed and the operating amount of theaccelerator operating member; the connection determining unitdetermining that the assistance from the third motor is required when apredetermined connection determination condition is met, thepredetermined connection determination condition is being a selection ofthe lowest speed range by the speed change operating member, or atractive force obtained from the output torque of the first motor andthe output torque of the second motor when the output torque of thefirst motor or the output torque of the second motor required forgenerating the required tractive force is equal to or greater than apredetermined upper limit threshold, being less than the requiredtractive force; the connection determining unit determining that theassistance from the third motor is not required when a predetermineddisconnection determination condition is met; the predetermineddisconnection determination condition including a selection of a speedrange other than the lowest speed range by the speed change operatingmember, and a tractive force obtained from an output torque of the firstmotor and an output torque of the second motor not being less than therequired tractive force; and the motor switch control unit controllingthe motor switching mechanism so that the third motor is set to thedisconnected state when the rotation speed of the third motor is greaterthan a predetermined upper limit value even when the predetermineddisconnection determination condition is not met.
 17. The work vehicleaccording to claim 2, further comprising a mode switching mechanism forswitching a driving power drivetrain of the power transmission devicebetween a plurality of modes; and a speed change operating member forselecting a speed range that defines an upper limit of a vehicle speed;when the third motor is in the disconnected state, the motor commandvalue determining unit determining the command value of the torque orthe rotation speed for the third motor so that the third motor is set tothe standby state at a predetermined rotation speed in accordance withthe selected mode and the speed range.
 18. A control method for a workvehicle comprising: an engine; a hydraulic pump driven by the engine; awork implement driven by a hydraulic fluid discharged from the hydraulicpump; a travel device driven by the engine; and a power transmissiondevice that transmits driving power from the engine to the traveldevice; the power transmission device including an input shaft; anoutput shaft; a gear mechanism that has a planetary gear mechanism andthat transmits rotation of the input shaft to the output shaft; a firstmotor connected to first rotating elements of the planetary gearmechanism; a second motor connected to second rotating elements of theplanetary gear mechanism; a third motor for assisting the first motorand the second motor; and a motor switching mechanism configured toselectively switch a connected state in which the third motor isconnected to the first motor or the second motor and a disconnectedstate in which the third motor is disconnected from both the first motorand the second motor; the power transmission device being configured sothat a rotation speed ratio of the output shaft with respect to theinput shaft is changed by changing the rotation speeds of the firstmotor, the second motor, and the third motor, and the method comprisinga step for determining whether assistance from the third motor isrequired or not; a step for controlling the motor switching mechanism sothat the third motor is set to the connected state when assistance fromthe third motor is determined to be required; and a step for controllingthe motor switching mechanism so that the third motor is set to thedisconnected state when assistance from the third motor is determined tonot be required.